Handle experimental feature dependencies correctly, completely, DRY

[roberth]

Describe the bug

While working on the fetchTree docs, #9258 we found two issues:

• The flakes feature did not automatically enable fetch-tree
• The fetchTree documentation only mentions fetch-tree instead of: fetch-tree or flakes.

We might violate DRY a bit in #9258 to get the docs to render first. This issue describes the tech debt.

Note that we don't have to do elaborate graph concepts in code; just an interface that gets the correct info for these two use cases, and an implementation that gets the job done with minimal complexity.

Expected behavior

Completeness, soundness, DRY.

nix-env --version output

Additional context

• Found document fetchTree #9258

Priorities

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[Ericson2314]

The flakes feature did not automatically enable fetch-tree

I am confused. I made hack with a fixme on master in src/libutil/config.cc in f0adb72, then 12953b9 removes the FIXME but leaves the hack

We should definitely make a better non-hacking version of this: declarative experimental feature implications, I think. But we shouldn't need another hack!