Look up `dub.selections.json` files in parent directories too (#2272) ...

* Look up `dub.selections.json` files in parent directories too

In case the root package directory doesn't contain a `dub.selections.json`
file, look in parent directories too and load the first (deepest) one.

This allows using a 'central' `dub.selections.json` file for a repository
containing multiple dub projects, making it automatically apply to all
builds in that source tree if located in the repository root directory
(unless a local `dub.selections.json` overrides it).

Such an 'inherited' selections file is never mutated when running dub for
a nested project, i.e., changes are always saved to a *local*
`dub.selections.json` file.

Tests: Implement the ability to mock the registry too ...

So far tests have limited themselves to the local state,
but we want to expand them to test the behavior of dub
when fetching packages, so we need a convenient way
to add packages to the registry.

FSEntry: Set a default time for all entries ...

Otherwise we are not able to write the entries in a Zip file,
as we get an exception from std.zip.
This is a workaround and a proper fix would be to implement
a Clock system.

Work around file locking dependency in PackageManager ...

The `dub.internal.utils : lockFile` function does not land itself to mocking quite well,
as it exposes a struct containing a `std.stdio : File` instance.
The path of least resistance, for now, is to access `store_` directly from
TestPackageManager by overriding `store`.

FSEntry: Throw an Exception when trying to read non-existent file ...

FSEntry.lookup can return null, and it would lead to a SEGV
with the attribute check, so add the forgotten check first.

PackageManager: Make lookup lazy again ...

We want to avoid loading the whole cache whenever lookup is called,
as most invocations of dub should result in a call to lookup,
since dependency resolution is much rarer than building with an
existing dub.selections.json.

Move the I/O part of Project.loadSelections to PackageManager ...

By doing so we can remove a sizeable amount of duplication as well
as the last `Dub.loadPackage` override. The resulting API is not
ideal, for example it doesn't allow to distinguish between parsing
errors and non-existent file, but will have to do for now.

Don't call `getPackageIterator` from `loadGitPackage` ...

`getPackageIterator` API forces us to do a refresh, which loads all
packages on the filesystem. However, for `loadGitPackage`'s use case,
we actually can simplify the check quite a bit.

Doing so means that we can now avoid doing a scan for projects
that use SCM-only dependencies.

Test Plan: On master, running the test (without the fix) triggers:
```
core.exception.AssertError@source/dub/test/base.d(790): Trying to access poisoned path: /dub/user/packages/poison/1.0.0/poison/dub.json
```
While with the fix, the test passes.

Let unittest use PackageManager.store ...

No need to mock, all filesystem functions have been mocked
by the PackageManager now.

VFS: Implement file attributes ...

To more accurately reflect the real filesystem,
we now support the access and modification times
for FSEntry.

Test that `add-path` packages take priority ...

This was not currently tested explicitly, and is currently
the reason why we still have the scanning behavior in the
PackageManager.
In order to remove the scanning behavior and have the PM
lazily scan packages, we first must add test to ensure
there will be no further regression.

Test: Make NativePath import public ...

So that tests can use 'NativePath' and 'toNativeString'.

Add the ability to re-invoke Dub with the same vfs ...

This will prove handy when writting tests that require multiple
invocation of Dub, such as idempotence tests, or things like
add-path which currently don't propagate their change through
Dub as they are one shot commands.

Mock IO in writeLocalPackageList / writeOverrides ...

Mocking IO is required for writeLocalPackageList,
as we want to test the 'add-path' behavior.
While it is not a big concern for overrides, as they
are deprecated and we are unlikely to add new tests,
it is easy to do, and already done for reading them.

Remove the need to mock PackageManager.refresh() ...

We can finally test the full features of the base function,
as we are now mocking the IO, not the logic.

PackageManager: Mock readText, do not mock load ...

This allows us to test the behavior of the base class much better,
by overriding the last IO function, readText.

Move Package.findPackageFile to PackageManager, mock ...

Moving it to PackageManager allows us to call PackageManager.existsFile,
allowing us to mock a lot more things.

Remove the need for intermediate `.zip` file in PackageSupplier ...

By using a different overload of `retryDownload`,
we return the data instead of writing to disk,
ensuring that we don't have a lingering file
in memory and reducing IO.

This follow the same approach for deprecations as previous
`PackageSupplier` deprecations: it's an inevitable, but
easily fixable, breaking change for classes that implement
`PackageSupplier`, but it's a proper deprecation for client
code, based on the assumption that all the classes implementing
`PackageSupplier` are in Dub.

Test: Directly mock writeFile/existsFile from PackageManager ...

This allows us to reduce the number of functions we override,
as we would like to test writeSelections and refresh directly,
not the copy of them we have.

Introduce FSEntry.removeFile, FSEntry.removeDir, refactor FSEntry.writeFile ...

Add the ability to remove directories and files,
and refactor writeFile to make this kind of function
easier to write.

Allow tests to call customCachePaths ...

This is now possible, as refresh has been mostly implemented.

Tests: Remove FSEntry.this dependency on TestDub ...

The dependency should always be the other way around (TestDub depends on FSEntry),
and this is an easy win to move.

Make FSEntry const and scope correct ...

This way we can use it in functions that are using
const/scope, like loadConfig.
As a minor improvement, the two `lookup` overloads
were moved next to one another.

Test: Move writePackageFile to UFCS, better document ...

This is another step in separating FSEntry from TestDub.

Merge both TestDub constructors ...

Those constructors had different but non-overlapping functionalities.
In the event one wanted to set one of the parameters passed to Dub
constructor but also initialize the FS, one was out of luck.

Tests: FSEntry.writeFile write to wrong directory ...

This was a subtle bug hidden by the fact that 'parent' exists
in the class. It was causing problems when calling 'print',
but luckily didn't affect anything else.

Test registry: Store version separate from Package ...

This will allow us to store full packages (FSEntry) instead of
the processed / already instantiated Package.

Introduce readText and make stripUTF8BOM private

Load dub.selections.json in unittest ...

Allow better testing by loading already resolved dependencies.

Move writing selections file to PackageManager ...

The PackageManager is the main source of IO in Dub,
and having it take responsibility for writing the
selections file reduces the number of special casing
we need to make when doing dependency injection.