Issue description

Nixpkgs' architecture is not the best. Issues:

• Leaks memory: too many cycles mean nothing can be GC'd
• overriding is gross: We should compose small parts into the final thing we want. "Modifying" the whole post composition is fundamentally anti-declarative

Potential solution

• I think we need to expose all the functions we callPackage on their own.
• As a middle ground, also expose the function to be fixed ("all packages") but no fixed point "yet"
• derivations shouldn't leak either inputs, but just return string (single output) or record of strings. Anything we want to expose should be done with passthru
• No .override .overrideAttrs .extend or similar. If you want something different, you need to rebuild the fixed point manually.

This is a radical shift that would make many "minor" overrides quite a bit more annoying, in addition to other breakage. But I think it would also put us on a much firmer, more sustainable foundation. Any idea how we get there incrementally?