feat: implement extras with conditional dependencies

[baszalmstra]

Uses conditional dependency support in resolve to implement extras. Result of some late-night hacking with @wolfv .

Extras a written as condition dependencies, e.g. if a package foo has an optional dependency group called bar

"foo": {
  "extra-depends": {
    "bar": ["numpy"]
  }
}

Then conditional requirements are added for foo in the form of: numpy; if foo[bar]. We add a special "extra" package that represents foo[bar]. So as a user depends on foo with bar (foo[bar]), numpy will also be selected.

This also adds extras in a different way. Extras are completely optional now, so if a user depends on an extra that doesnt exist the solver will happily continue. Its up to users of the library to warn if an extra is selected that is not actually available for the selected package.

[wolfv]

The benchmarks show a inconsistent difference of +-3% which seems fine to me!

[wolfv]

ugh, somehow the extras printing adds a single space after ]? I checked and it doesn't seem to come from this code.

[wolfv]

I think prefix-dev/resolvo#164 should fix this.

[baszalmstra]

I updated the resolvo dependencies, merged main, and fixed the thing you find. This looks good from my perspective @wolfv

[wolfv]

Awesome stuff! I think now I need to rebase my PR in #1545