`nix derivation add`, and its C API counterpart, now works a bit closer
to `builtins.derivation` in that they don't require the user to fill-in
input addressed paths correctly ahead of time.
The logic for this is carefully deduplicated, between all 3 entry
points, and also between the existing `checkInvariants` function. There
are some more functional tests, and there are also many more unit tests.
Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>
Co-authored-by: edef <edef@edef.eu>
