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.

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.

Move the code for collecting package information to a separate method.

Reorder code so that local functions come before the high-level algorithm.

Consolidate package related information in getPackageConfigs into a single struct.

Remove unused function.

Refactor removeConfig with filterInPlace.

Fix parameter naming convention.

Avoid visiting packages more than once in checkPacksRec.

Avoid linear time edge list construction. ...

Uses a set to determine redundant edge entries instead of iterating through the list.

Reduce allocation overhead in determineDependencyConfigs.

Avoid linear lookup and name allocations in Project.getDependency.

Avoid linear time haveConfig.

Use package indices instead of names or Package instances to identify packages. ...

Avoids costly AA lookups and string comparisons.

Avoid re-allocating allconfigs_path in determineAllConfigs.

Avoid re-allocating the AA for isReachableByAllParentPacks for every call.

Eliminate edges in the configuration graph in-place. ...

Avoids reallocating the edges array every time the configuration resolution eliminates a configuration.

Cache various package information to reduce GC allocations in getPackageConfigs. ...

Package.name and Package.getAllDependencies both need to reallocate their return value for every call, which results in heavy GC pressure during the configuration resolution algorithm.

Return early from determineDependencyConfigs if possible ...

The only package that can have a config already created is the root
package, and only if the `config` parameter isn't an empty string.
Every other package is in the `configs` array iff it was added by
a previous call to this function. The exact same configurations
will be found if this function is called again for the same (p, c)
pair and it will do needless work that won't modify anything.

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.

Remove an unused JSON serialization call ...

This was completely unused in the function.

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.

Make 'Dependency.init' invalid and align Any/Invalid property ...

VersionRange has Invalid and Any while Dependency has invalid and any.
One has to go, and it's going to be Dependency's.
In addition, make sure that the 'init' state of Dependency is a known state.

Adapt PackageManager.get[Best]Package to PackageName API ...

This paves the way to be able to automatically return subpackages.

Adapt loadSCMPackage to PackageName API

Remove PackageName alias this to toString ...

The alias this was originally added to make the transition
less painful, however due to an overload resolution bug,
it makes it harder at the moment, as thje compiler prefers
calling the deprecated `string` overloads over the
`in PackageName` ones. So remove it and a few hacks along
the way.

Adapt SelectedVersions to PackageName API ...

Adapting to the PackageName API meant relaxing the input
a SelectedVersion method can accept, as we now handle
the subpackage part gracefully (ignoring it), instead
of potentially running into issue.

Expand selections file handling to handle new versions ...

Up until now, the selections file has had a `fileVersion` in it,
but that number was always stuck to 1.
In order to make it possible to handle more than one version,
a bit of scaffolding needs to happen in the config parser.
This lays the ground for a version 2 of the selections file.

Introduce PackageName, to fully type a package name ...

Package names being passed around in the Dub codebase are string,
which is problematic as it suffers from validation / semantic issues.
Validation because we cannot impose restrictions on characters
or length easily, and semantic because some functions expect a main
package while some would accept a subpackage. Introducing a type
will remedy both those issues.

Allow building deprecated modules without deprecation warnings ...

The compiler issues a deprecation warning only on a
"non-deprecated" -> "deprecated" transition.  As such, building or
testing Dub packages which have deprecated modules when a
dub_test_root.d file needs to be generated will cause deprecation
warnings.

This in turn makes it impossible to use `buildRequirements
"disallowDeprecations"` in this situation.

Fix this by simply making the auto-generated root
module (dub_test_root) deprecated. As it is the compiler's entry
point, the module itself will not cause a deprecation warning, and it
importing deprecating modules will not cause deprecation warnings (as
there is no longer a transition of deprecation state).