stabilize the cache directory structure

Add unittest to test collection of C sources and headers

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

fix #2587

fix verbose logging crashing because of asserts

review adjustments

fix typo(s)

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

reduce dependency on the working directory ...

Improves library API usage and bugs with the build generator

Fix various DDOC warnings

Use a central build cache for all build artifacts ...

Currently the central build cache is not configurable,
but as most build artifacts end up in `~/.dub/packages/` anyway,
this will be a positive change for users.

Rename UserConfiguration to Settings ...

The latter is more accurate, as we can have system settings
of type UserConfiguration.
This is also a push towards more consistent naming within dub,
as configuration is primarily used for package recipe 'configuration' field.

Move user settings to dub.data.settings ...

By moving it to its own module, we can ensure it's almost a leaf module.
We can also extend unittests to check different scenarios
without relying on integration test.

Rename localRepository to userPackages ...

The 'repository' terminology is confusing, and needs to be interpreted
from the PoV of the PackageManager to make sense. It is likely that
a naive read of this name would lead one to think of git repository,
as opposed to the repository where packages are stored.

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.

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.

Dub: Use removeFile instead of std.file.remove (#2516) ...

In the end it calls the same function, but this allow us
to consistently inject dependencies.

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 yet another dub.remove overload using force_remove

Dub: Deprecate changing the rootPath ...

It's an essential part of the setup, and too many properties are derived
from it that we don't want to let the library user the ability to mess with it.

Avoid throw expression - fixes compilation on LDC 1.30.0.

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.

Remove PlacementLocation information from Dub.fetch ...

Let it rely solely on the PackageManager instead.

PackMan: Deprecate overload leaking location path ...

Currently, a lot of the internals of the PackageManager are leaking.
The getPackage overload accepting both a version and a NativePath
is an example of such a leak, and looking at the usage sites
show that things can be simplified by taking a PlacementLocation,
which is a higher level representation of the path.

Move PackageManager.getPackagePath to Location ...

It is more suited as a Location method, as it returns a path based on PackagePath.
This also allows us to reduce the leak of information about Location
happening in dub.

Dub: Do not re-load already loaded tool Package ...

Instead of giving the path to the Package we just loaded,
just give it the Package we just loaded.

Dub: Remove redundant variable m_projectPath

Remove PackageManager deprecations from v1.20.0 ...

Those deprecations have been introduced in v1.20.0,
so if we follow the same 10 releases policy as DMD,
we are good to have them removed in the next release.
Deprecated in 48a26db

Dub: Use UpgradeOptions.dryRun instead of printUpgradesOnly ...

The latter is said to be deprecated, and this is the only usage.