Adapt DependencyResolver to PackageName API

review adjustments

Move environment read outside of DependencyResolver ...

Reading environment variables from random places in the code makes
the flow harder to follow, and unittesting harder.
This moves one environment reads to the `dub` module,
which performs many of them already.

Trivial: Change max_iterations type to ulong

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

Remove unused selective imports. These are private imports of symbols that do not occur elsewhere in the file, so they are safe to be removed.

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

In support for dlang/dmd#11000

Replace `hash_t` and `sizediff_t` with `size_t` and `ptrdiff_t`

Fix unresolved optional dependencies causing other dependencies to be skipped.

Add unit tests for dependency resolution error messages.

Implement a recursion based dependency resolution algorithm. ...

Although this is rather expensive in terms of memory allocation and stack usage, it does a much better job at reducing the number of combinations that have to be tested.

Rename ConfigEntry to PackageConfigs and avoid GC allocation.

adjust code style to request

Fix #1037

Remove compatibility code for DMD frontends < 2.067.x.

Defer error message generation to the end of the dependency resolution process.

Put a hard limit in the number of dependency resolution iterations. ...

This avoids the process getting stuck in the resolution phase "indefinitely", so that for example CI runs don't get stuck anymore. It also communicates the problem and potential resolutions more clearly. If the DUB_NO_RESOLVE_LIMIT environment variable is set, the resolution process will run without this limit.

Small optimizations in DependencyResolver.

Fix diagnostic output for dependency resolution failures.

Fix recursive graph traversal. Fixes #923.

Don't ever reset the any_config flag to false. Fixes #896. ...

Avoids a bad error message indicating a missing *package* when it really is only missing a suitable *version*. Occurs for dependencies that have multiple incoming references, with an unsatisfyable version reference coming form the root package.

Fix second failing case of #803.

Fix naming of rootPackage() helper.

Avoid repeated addition of the same dependency in DependencyResolver.

Fix dependency resolution for unsatisfyable optional dependencies. ...

If another (unchosen) dependency in the graph had a required dependency to the unsatisfyable version, the resolution process failed, even if not choosing the dependency would have been valid.

Add additional unit test for optional dependencies.

Improve error message for unsatisfiable root dependencies.

Filter out invalid dependencies before starting the resolution process. ...

Package versions for which it is not possible to get an actual package description (e.g. due to non-existent paths or semantic errors in the package description) are now excluded from the dependency resolution process. Before this, they could erroneously be picked, resulting in an malformed dependency tree.

Fix purging of optional dependencies when sub packages are involved.