Add unittest to test collection of C sources and headers

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

fix #2587

fix typo(s)

replace std.variant with std.sumtype (#2550) ...

Variant is quite big and runtime heavy, with sumtype we already cover
wrong type cases at compile time.

Reduce dependency on `std.file` (#2547)

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.

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.

Minor: Align dub.recipe.selection style with dub.recipe.packagerecipe ...

During review of the PR that switched PackageRecipe parsing to use Configy,
WebFreak pointed out that having an internal, private structure used for
parsing YAML field was preferable to having it at module level.
This aligns the names and the approach used with the one in PackageRecipe.

Trivial: Fix typo in documentation

Use configy library to parse dub.json file ...

This will provide a vastly superior UX to the users:
- They will be warned about typos;
- The type information provided will be more accurate and always include file/line infos
  (e.g. no more 'Invalid SemVer versions');
- We can expose a better recipe struct to library users on the long run,
  by separating the actual data DUB uses from what it reads;
- We can be more descriptive about errors / intent, for example by using `SetInfo`;

Trivial: Remove test about the internals of Dependency ...

Now that Dependency is a proper SumType, this can never happen.

Integrate `dependencyBuildSettings` into dependencies ...

Because those are specified in `dependencies`,
they should be stored alongside them.
Unfortunately this will slihtly increase memory usage in the common case,
but unlikely to a point where it would be problematic.

This is technically a breaking change as a field is getting removed,
however this struct has been manipulated a few times in breaking ways,
and it's unlikely `dependencyBuildSettings` is being used outside of dub,
given how recent it is and how narrow its usage is.
However, as `dependencies` is likely to be widely used, a compatibility
type and an `alias this` are provided not to break client code.

Trivial: Add attributes and documentation to PackageRecipe ...

While the attributes aren't used yet, they are self-descriptive,
and they improve code readability.

Configy: Replace parseField with parseAs ...

This removes the need to keep the representation around and have
it stored in a field of the enclosing struct.

Deprecate Dependency.versionSpec ...

All places which assumed that Dependency contained a VersionRange have been culled,
so we can now start removing those properties.

SDL: Use visit instead of multiple if when parsing Dependency

Keep logging functionality out of the public API as requested in https://github.com/dlang/dub/pull/2124#issuecomment-813016939.

New logging module - initial functionality ...

The new module replaces dub.internal.vibecompat.core.log by using the
same function names, but implementing colored output and the left
fixed-width tag. Overloaded functions are provided to output colored
text.

Still missing initLogging() and non-TTY stdout/stderr detection

Use Configy to read dub.selections.json

Fix missing sanitizing of subpackage targetTypes ...

A regression introduced in #2277.

sdl: Simplify Dependency parsing ...

Don't warn on ignored version specification, json doesn't do this.
Also fully replace the instance instead of editing piece-meal,
it makes the code clearer.

Various code simplification around `dub.selections.json` (#2311) ...

* Deprecate unused Json constructor

* Trivial: Fix import order in dub.project

* Simplify DependencyResolver filtering

DependencyResolver is only instantiated from one place, and never re-used.
It makes more sense to set those variables in the constructor,
and avoid repeatedly calling a single function.

* Move file format to dub.selections.json to its own struct

Our goal is to replace the handcrafted JSON code with configy.

Add SumType support to packagerecipe : clone ...

clone does not support enum, but it can support discriminated enums.

Dependency: Move `spec` / git ref inside of `Repository` ...

The `Dependency` struct is logically acting as an `enum`,
with 3 discrete possibilities. However, some of the logic is
piggybacking on the (original) version logic.
`Repository` is one of them, and by separating things,
we can make stronger assumptions in `Version` and `Dependency`.

Remove leading hyphen in platform specs ...

It doesn't serve any purpose.

[add BuildSettingsTemplate.dup() method instead]

Move fix for issue 711 outside of JSON/SDL parser ...

Having the fix for this issue in the parser means adding non-trivial logic.
It is easier to reason about to have this logic done as a post-processing step,
replacing the `autodetect` with the actually auto-detected version.
This also paves the way to have other `targetType` as default,
e.g. the `unittest` configuration should probably not be `library`.

sdl/json: Do not create dummy dependencyBuildSettings entry ...

Because if a user provide an object as a dependency,
such as `{ "path": "foo/bar", "optional": true }`,
we would create a dummy entry in the AA.