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.

Trivial: Remove an always-true if (#2313) ...

Since we are using a path-based dependency, dep.matches will always return true.

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`.

getDefaultConfiguration(): Accept a const BuildPlatform argument ...

Like `Package.getDefaultConfiguration()` already does. Fixes a minor
annoyance for reggae.

Use Dependency.any whenever possible

Merge remote-tracking branch 'origin/stable' into merge_stable ...

Conflicts:
	source/dub/dependency.d

Also use strict version matching for cached packages when determining the need to fetch a dependency. ...

Fixes #2262.

Fix dub_test_root imports for non-all-at-once compilations (#2288) ...

Which requires a regular `dub_test_root.d` filename *and* adding
the containing directory to the importPaths.

Make `dub describe -c unittest` auto-generate dub_test_root.d if required
Except with `--annotate` for a dry run.

Add a command line option for upgrading all sub packages. ...

Allows to upgrade all path based sub packages of a package, in addition to the package itself, in one go. See also #1245 and #2284.

Handle unittest config special case in Dub.generateProject()

Refactor Dub.testProject() to Project.addTestRunnerConfiguration() ...

With some slight functional changes in Dub.testProject() - not
resetting m_project and creating the generator with the final
project.
And some minor log output changes.

Fix #1856 - enable building the special test runner config

Posix: use /etc/dub/settings.json if in /usr ...

Fix #1472

/etc/dub/settings.json overrides /usr/etc/dub/settings.json (as
suggested by the `man hier` manpage)

Fix issue 2051 failing CI

Remove line 'Excluding package.d blah...' from default dub build

Fix #2141 - When used as a library, the default compiler is different from the expected. (#2158)

Add environment variables to BuildSettings (#2121)

only show package.d warning once

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

fixed a bug with DFLAGS environment variable and cached generated unittest

Add lowmem for settings

Add the main source file directory to import paths ...

else dub doesn't find it because the single file package do not have the default package directory structure

In single file packages the main source file shouldn't be ignored ...

because it represents all package by definition

Trivial: Clean up trailing whitespace

Trivial: Remove redundant import ...

This is already imported at module scope.

Improved ldc cross compile: ...

The folder `.dub/obj` has been moved to `.dub/build-*/obj` for an independent cross / parallel build process.
Now it is possible that two processes can build the same project with different configurations.

Fix #2051 "Running unittests from dub single file packages fails" (#2052) ...

Performing test command dub imports every source file as module to build the project excluding the main source file. But this excluding may fail and in this case dub try to import the main source file as module. Single file package building fails because the main source file does not belong to any import path. Now both the current file and the main source file paths are compared in relative form to prevent importing the main source file as module.

Use DC env variable as the default compiler

Minor: Remove superfluous if in Dub constructor ...

getPackageSuppliers already handles SkipPackageSuppliers.none correctly,
no need for an extra check.

cache the generated test main file: dub_test_root.d ...

these changes prevent dub from rebuilding a project every time it runs a unittest.