Source roots result in /nix/store/<hash>-<hash>-source

[edolstra]

Describe the bug

In a flake with a derivation attribute like

src = ./.;

the tree gets copied into the store as /nix/store/<hash1>-<hash2>-source. This is because (pre lazy-trees) the basename of the tree is /nix/store/<hash2>-source.

This double-copying is obviously undesirable, but we can't fix it without a breaking change (i.e. without changing derivation paths).

On lazy-trees, we can at least avoid writing /nix/store/<hash2>-source to disk, but we do need to hash the input to compute <hash2>. (See c5ae41d.)

Steps To Reproduce

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Expected behavior

A clear and concise description of what you expected to happen.

nix-env --version output

Additional context

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[KiaraGrouwstra]

we could already improve on the user experience here by warning the user to avoid such paths.