New API bits for pip: plugin loading, and wheel file supported check

[mgorny]

Quite a big and not yet complete: I still need to add more checks and tests.

1. With this change, we have three metadata classes:
   • DistMetadata for distribution METADATA files
   • VariantPyProjectToml for pyproject.toml
   • VariantsJson for variants.json
2. All of them derive from shared VariantMetadata model that covers the common bits, plus have their own extra fields.
3. PluginLoader now has a constructor that accepts any VariantMetadata subclass and loads plugins from it.
4. For get_variant_hashes_by_priority(), plugin_loader is now optional, so you can either pass your own or let it load plugins from variants.json. This is similar to Variant Plugin Auto install #86 but it preserves the ability to provide custom loader, which will probably be useful for pip isolated envs.
5. There is a new check_variant_supported() function that can be used to check whether a single variant description can be installed — now used by pip when installing local files.

The classes are pretty robust now, and I think we will use them in the future to reimplement some of the commands that now inline metadata reading and writing.