Allow evaluating local flakes directly

[Atemu]

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Currently, when you do something like nix build .#hello inside a local Nixpkgs checkout, the entire needs to be dumped into the Nix store.

That takes quite a while even on an SSD and wastes erase cycles too.

This behaviour is especially problematic in combination with bash completion as it calls nix for everything separately. Bash completion is unusable with local flakes.

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Local flakes should be evaluable directly just like regular local Nix expressions are.

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The state of the directory could at least be cached but that's not a great solution.

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https://discourse.nixos.org/t/troubles-adopting-the-new-nix/12870

[edolstra]

Duplicate of #3121.