Allow overriding the follows of a transitive input without overriding its flakeref

[edolstra]

Describe the bug

E.g. an override like

inputs.foo.inputs.bar.inputs.nixpkgs.follows = "nixpkgs";

implicitly sets inputs.foo.inputs.bar to flake:bar, leading to an unexpected error like

error: cannot find flake 'flake:bar' in the flake registries

Expected behavior

The original flakeref for the bar input in foo's flake.nix should be respected.

nix-env --version output

2.16.0

Priorities

Add 👍 to issues you find important.

[Kha]

This looks like a duplicate of #5790. I just checked that #6621 applies cleanly to both current master and lazy-trees. I haven't rerun the tests yet though.

[gilice]

Probably also related to #6986

[citriqa]

@Kha it just applies cleanly because that's a noop, since it's already & still applied. a revert cannot be undone be re-applying - easy to overlook (and git of course doesn't mention that the action had no effect...)
my attempt to un-revert in the lazy-trees branch unfortunately resulted in a strange boehm-gc allocation failure that i haven't yet figured out.