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.

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`.

Make Project.addTestRunnerConfiguration() accept a const GeneratorSettings argument

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

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

[add BuildSettingsTemplate.dup() method instead]

Project.addTestRunnerConfiguration(): Don't accidentally modify base config's buildSettings ...

This most likely doesn't affect dub itself, as it only works with
a single configuration at a time. The problem has surfaced with reggae
though (which can use multiple dub configurations at a time) - the
`mainSourceFile` was removed from the base `application` config too.

Project: Remove dead m_packageSettings var and related func ...

git blame points to 2014, and it seems to be the ancestor of dub.selections.json

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.

Don't ignore targetName in non-executable unittest configurations

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

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

Don’t call writeln in generated code when measuring coverage and indent generated code

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.

Resolve WebFreak's comment about processVars empty

Change description of injectSourceFiles to inject-source-files ...

Co-authored-by: Jan Jurzitza <gh@webfreak.org>

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

expose the implemented describe pre/post-run cmds

substitute commands in dub describe output

move command variable substitution to execution ...

This is needed as commands that need to be run need to have generator
settings available before running to avoid inconsistencies with
environment variables passed to the actually ran program and variables
used in substitution.

Fixes #2192

Fixes pre/postGenerateCommands inconsistently using dub environments
(execution was substituted including buildEnvironments, but shell cmd
did not get the build variables for substitution) - now both get
the build variables. Solved by removing code duplication. (env building)

This commit changes the behavior of `dub describe` so that describing
commands settings no longer substitutes variables. Before, known
variables would have been replaced in the output and on unknown
variables `dub describe` would fail with an error
`Target 'x' doesn't exist`

Now exposes some previously private project.d processVars functions

Fix issue 2051 failing CI

fix CLI --print-builds implementation

Fix --data-list error message with dub describe (#2195) ...

It said you can also use --data-0, but that actually will not make it
work. You need to still specify --data-list and optionally add -data-0
to make it 0 separated instead of line separated.

Quotation marks added to error message

Add environment variables to BuildSettings (#2121)

Fix #2046 - Dependencies specified with a path ignore optional settings

Refactor import. inet.path is publicly imported from inet.url, but only symbols from inet.path are used.

Disable warning that import path list is empty in single file package mode

Don't export DFLAGS & LFLAGS to nested invocations by default

Support dependencies as git url with exact commit (#1798) ...

* Support dependencies as git url with exact commit

* Compare hashes for SCM

Remove foreach variable shadowing

Replace calls to NativePath.toString() with .name ...

This has been deprecated in vibe-core and only triggers with 'library-nonet'.