Fix ubyte[] download overload by avoiding a cast ...

We were using the default template parameter, `char`,
and as the documentation for `get` states:
> If asking for char, content will be converted from
> the connection character set (specified in HTTP
> response headers or FTP connection properties,
> both ISO-8859-1 by default) to UTF-8.

However we are downloading a binary so converting
it to UTF-8 will fail. Especially if we intend
to cast it to `ubyte[]` afterwards.

Trivial: Print the Exception message in retryDownload ...

This makes the source of the issue more obvious when using
the verbose flag.

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.

Remove FileInfo.timeCreated ...

As it is not available on all platforms, we can't really rely
on it for code, and it is indeed not used anywhere.

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.

Merge pull request #2866 from Geod24/mlang/Simplify1 ...

Trivial: Simplify PackageManager's loadInternal

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.

Merge pull request #2865 from dlang/fix_issue_2691_reboot ...

Properly fix issue #2691 by reverting to the original scanning behavior

Simplify condition.

Trivial: Simplify PackageManager's loadInternal

Properly fix issue #2691 by reverting to the original scanning behavior. ...

Reverts to the scanning/warning behavior that was in place before the warnings for the new package directory structure were implemented.

Revert "Fix #2691: Use same structure for managed and unmanaged path" ...

This reverts commit 1a53049.

Remove an unused JSON serialization call ...

This was completely unused in the function.

Tests: Use TestDub FSEntry constructor for more accurate tests ...

The previous approach added Packages in the PackageManager directly,
completely sidestepping the scanning the PackageManager does,
which is a behavior we absolutely want to test.

By setting up the filesystem before we instantiate our `TestDub` instance,
we can remove a bunch of exceptions / overrides, and get a much closer
behavior to the final product.

Tests: Fix vfs paths on Windows ...

With our versioned path implementation, we have the following issue:

assert(NativePath("/foo").absolute); // Passes
assert(NativePath(NativePath("/foo").toNativeString()).absolute); // Fails

However, instead of fixing the implementation, we provide
a different root for the VFS, which is arguably simpler but
also more correct.

Hoist imports and remove versioning in FilesystemPackageSupplier ...

Hoisting imports to have them all in the same place.
Also remove a version statement, as the client code should not
care if vibe-d is being used and just import the internal module.

Clear up unprinted deprecation for overrides ...

This code should have triggered a deprecation, however it didn't
get printed for some unknown reason. Turning on '-de' leads to seeing
it. The fix is fairly straightforward and doesn't affect the code.

Use the proper constructor for PackageDependency ...

This code was using a deprecated constructor, but as it was in lambda,
it didn't trigger a deprecation message, however '-de' points us to
the issue.

Tests: Fix writeFile implementation with vibe-d ...

This was triggering an assert error with vibe-d path.
The recursion logic was a bit murky, so the function
is now split in 3 different part and better documented.

Make all overloads of download private ...

Since this is not used directly anywhere inside of dub,
make it private so it can be later refactored.

Tests: Simplify loadSCMPackage implementation ...

Now that we can override the git clone operation and load
from the virtual filesystem, we can remove a lot of the
duplicated logic. This is currently hardcoded to a dub.json,
but future iterations can store an FSEntry representing the
full package.