Motivation for this change

This handles a specific case, when "path" type was supplied to types.path
option. Previously it was converted into store path, because it was used
in other derivations:

nix-repl> builtins.toJSON /etc/nixos/configuration.nix
"\"/nix/store/wd12lm99vp1syi3ywala46w4zhnvk83z-configuration.nix\""

So making users.users.alice.home = /home/alice; would convert /home/alice
directory into store path.

I can't think of situation when this is what is expected. State dirs and
password files could have been converted into store paths as well.
Having state dir on ro mount would probably result into error, but
password files were simple enough, so could easily be copied into
/nix/store and nobody noticed.

On other hand, we can't cast types.path to string always, because
in many places package is provided instead of path. It worked because of
silent casts... I'll leave a TODO for future contributors.

Reported in https://discourse.nixos.org/t/re-use-existing-home-migrating-from-ubuntu/2353

PS. I don't know how to write automated test for this, because it involves "paths", which are awkward to work with in a reproducible manner. I've tested locally with a local "path".

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.