Add unittest to test collection of C sources and headers

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.

Remove leading hyphen in platform specs ...

It doesn't serve any purpose.

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.

Build{Options,Requirements}: Use a common templated struct ...

A bit less copy-pasta is a small but appreciable win.

Implement injection of source files from dependencies into parent final binaries (executed/shared libraries)

Add environment variables to BuildSettings (#2121)

Add dependencyBuildSettings to BuildSettingsTemplate

Use Dependency Json facilities

Replace occurences of 'in ref' with 'const scope ref' ...

In support for dlang/dmd#11000

Remove some unused variables ...

There are plenty left, but those ones didn't seem to have any side effect.

Change toolchainRequirements fields to Dependency. ...

Also removes some API clutter from Compiler and PackageRecipe.

Merge pull request #1571 from rtbo/requirements ...

toolchainRequirements recipe support
merged-on-behalf-of: Nicholas Wilson <thewilsonator@users.noreply.github.com>

toolchainRequirements recipe support

extraDependencyFiles attribute added

Added experimental build settings to filter applicable version identifiers ...

Those settings allow to filter version identifiers and debug version identifiers
used during building a package. This can improve the build cache efficiency for
libraries that are used by multiple applications with different version
identifiers.

- only merge filters from source dependencies when the
  dependent has an explicit filter list as well

Pre/post run commands added

Revert "implement "minDubVersion" field"

implement minDubVersion check ...

fix #1531

Eliminate use of Json.opDispatch. ...

(cherry picked from commit bf65c40)

Make internal function in dub.recipe.json private.

Serialize -ddoxTool in toJson.

Write "subConfigurations" field in JSON recipes. Fixes #745.

allow to specify ddox tool ...

- can be used to customize the ddox tool (and it's output)
- assumes similar commands/behavior

Change Build(Options/Requirements) to BitFlags.

fix selective imports and FQN symbols

Fix warnings on DMD master. See #541.

Apply patch by ketmar: remove comma expressions and fix property syntax. ...

Also contains two minor additional style fixes.

Fix warnings and errors on git master