Deduplicate source path fetching based on predictable fingerprint, using a weak_ptr cache

[roberth]

Is your feature request related to a problem? Please describe.

I'll use a flake example, but this applies to anything that uses evaluation-time fetchers such as fetchTree.

When invoking a command like the following, nix-darwin is specified twice.

$ nix run \
   "/home/user/h/nix-darwin?dir=modules/examples/flake#darwinConfigurations.simple.config.system.build.vm"
   --override-input nix-darwin /home/user/h/nix-darwin
   --override-input nixpkgs git+file:///home/user/h/nixpkgs
warning: Git tree '/home/user/h/nix-darwin' is dirty
warning: Git tree '/home/user/h/nix-darwin' is dirty
warning: Git tree '/home/user/h/nixpkgs' is dirt

As you can see, nix-darwin is processed twice.

Probably this could be worked around by avoiding dir in this case, but in general it seems worthwhile to deduplicate source paths internally.

Describe the solution you'd like

Deduplicate source paths based on fingerprint (when a fingerprint exists), before constructing a new instance, before fetching it to the store. Use a weak pointer map so that the source paths can be released when no more true references are held.

Describe alternatives you've considered

Additional context

Installables should also be deduplicated, but this is a different problem. (#9724)

Priorities

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