Minimal builtin provider support + validation improvements

[mgorny]

Add support for builtin providers in the models; API functions reject them as unsupported.

Fixed some random issues I've found while working on this:

1. Trying to export build-requires to variant.json will raise an exception now.
2. All API functions now only load providers that are actually used.
3. Variants without providers specified result in validation error rather than being silently ignored as incompatible.

[rgommers]

This all looks sensible from a reading of the diff, and in line with the latest version of the PEP draft from what I can tell without more detailed testing.

I'll note that I am unlikely to have any time to review in detail in the next two weeks, and I wouldn't wait for me either way - I haven't touched this code a lot.

[DEKHTIARJonathan]

I'll try to give it a look - I'm on PTO until Tuesday

[mgorny]

I'll note that I am unlikely to have any time to review in detail in the next two weeks, and I wouldn't wait for me either way - I haven't touched this code a lot.

Don't worry, I'm mostly adding all of you as FYI.