Only check if the Package is managed by the Location in lookup ...

Previously, we were calling back into the PackageManager to check
if the Package we were looking at was managed.
However, as we are only iterating on Packages that are in fromPath,
we should be able to rely on the Location.isManaged property,
instead of PackageManager.isManagedPackage, which just calls
Location.isManaged for each Location.

Trivial: Add a isManaged property to Location ...

Since isManagedPath is only used from PackageManager,
we want to move to a more fine grained approach,
starting with the Location.
At the moment the Locations iterate over all the Locations,
via the PackageManager, when they could realistically
just check if the Package they are dealing with is managed by them,
as it would be a bug to have a Package from one Location passed
to another.

Revert "Simplify PackageManager's Location.lookup" ...

This reverts commit 44852b8.

Make PackageManager.load non-const ...

It was made const to satisfy PackageManager.addPackages, but that
method actually takes a field of the PackageManager as ref parameter,
so it can never be called from a 'const' context, and while implementing
'TestPackageManager.load', it was found that 'const' wasn't suitable.

Simplify PackageManager's Location.lookup ...

The VersionMatchMode is a property of the Location,
not a property of the package, hence we can make
use of a Location attribute to get it,
removing a dependency on the PackageManager
and streamlining the code.

Deprecate `Package.load`, use `PackageManager.load` instead ...

In order for the dependency injection approach to work,
we need to be able to override every single instance of
`Package.load`. We could not override `Package.load` itself
as it is a static function.
However, in the process of adding support for path-based dependencies
to the test suite, it became obvious that the functionality offered
by `Package.load` is actually better suited for the `PackageManager`.
Deprecating `Package.load` confirms this: Not a single call was outside
of `PackageManager`, and no extra parameter had to be provided anywhere
to make this work.
This also nicely simplifies the `TestPackageManager` implementation,
as we can now overload `load` and let `getPackage` alone.

Test framework: Return fully typed TestPackageManager from TestDub ...

Using covariant return type, we can avoid the need to change the base class,
and leave all the overloading in the child class.

Add unittest framework and some dependency tests ...

Dub has always been notoriously hard to test due to the amount of IO it does.
Over the past few years, I have progressively refactored it to allow
dependency injection to take place, which this finally put into action.
By overriding the `PackageManager`, `PackageSupplier`, and a few strategic
functions, we can start to unittest Dub's behavior solely in unittests.
This should hopefully tremendously helps with adding regression tests
for the package manager side of Dub (the build side still need work
to have similar capabilities).

PackageManager: Make field protected, not private ...

This make implementing a different PackageManager possible.

Deprecate string-as-version PackageManager.getBestPackage overload ...

Because using a typed Version or VersionRange is better than using
a string and we want to encourage version validation to be done
earlier than later.

Remove inconsistent error on missing subpackage path ...

The behavior of the code was slightly puzzling: if the package path did
not exist, we could log an error and continue, but if it existed but
was empty, we would error out. To be consistent, simply always error out.

PackageManager: Store packages hierarchically, by version ...

As explained in the changelog, this hierarchy makes much more sense
than the current one, and will make it possible to read the version
from the directory name.

Merge remote-tracking branch 'upstream/stable' into merge_stable

PackageManager: Don't use deprecated interface to PackageOverride ...

This should trigger a deprecation message with DMD, but doesn't.
On the other hand, it triggers a linker error on GDC.

review adjustments

fix typo(s)

Replace all instances of mkdirRecurse with ensureDir ...

All usages actually want to ensure the directory is present,
and don't care about having a separation between existence check
and creation.

Merge pull request #2526 from dlang/add_path_workflow ...

Fix add-path based development workflow

Fix version match rules for add-path based packages.

Move foreign code from global scope to dub.internal. ...

Avoids conflicting with the actual dub packages when working with dub as a library. Also, even if just by convention, it ensures that dependent code doesn't rely on this code as a public API.

Unify reading and writing files under a single API ...

This removes the range API and instead uses Appender,
while using a limited and centralized set of functions
to read/write from files.
The end goal is to permit dependency injection,
which is a pre-requisite to extending the test-suite.

PackageManager: Load packages lazily on dub build ...

A problem of the current design of dub is that all locations are scanned,
regardless of what action is being performed, often multiple times.
With this change, a regular `dub build` on a project with all dependencies
resolved will no longer loads all available package, which can substantially
speed up development.

Document PackageManager.getBestPackage, deprecate Dependency overload ...

The Dependency overload is not needed, as calling getBestPackage
with an SCM / path dependency does not make sense.

Deprecate `PackageManager.refresh(true)`, only expose `refresh()` ...

It is not actually used, as all call sites but two use false as an argument.
The two calls using true are inside a constructor, meaning there will be no
pre-existing packages, so whether true or false is passed will have
the same effect / result.

Do not use Phobos' sumtype, use dyaml's versioned one ...

So that we can compile with older versions of the frontend,
currently v2.097 instead of v2.098.

Fix zip symlink problem.

Fix package scanning order. ...

Packages registered using add-path need to come before cached packages.

PackMan: Provide an overload of storeFetchedPackage that doesn't expose Location ...

This way we keep the internals of PackageManager within the PackageManager,
instead of having external classes be aware of the paths we use.

PackageManager: Only iterate on internal fromPath array, not everything ...

getOrLoadPackage is only used for path-based dependencies,
which are only stored in m_internal.fromPath.
We can thus reduce the iteration by only using fromPath.
If someone had a path-based dependency that pointed to a version-based package,
it would lead to this package being loaded twice, but that's not really a concern,
as it would not lead to problem either.

Remove PlacementLocation information from Dub.fetch ...

Let it rely solely on the PackageManager instead.