Use correct parent outPath for relative path inputs (backport #13170)

[mergify]

Motivation

Ensure relative path inputs are relative to the parent node's actual outPath, instead of the subtly different sourceInfo.outPath.

Without this, nested relative-path inputs within the same repo end up repeating the parent's relative path segment:

trace:
subdir for root:
flakeDir for root:
(no parent)
outPath for root: /nix/store/s4j8hchzds1x57hirv8y7p19nl44kqwr-source
sourceInfo.outPath for root: /nix/store/s4j8hchzds1x57hirv8y7p19nl44kqwr-source

trace:
subdir for nestedFlake1:
flakeDir for nestedFlake1: /./nested-flake1
parentNode.flakeDir for nestedFlake1:
outPath for nestedFlake1: /nix/store/s4j8hchzds1x57hirv8y7p19nl44kqwr-source//./nested-flake1
sourceInfo.outPath for nestedFlake1: /nix/store/s4j8hchzds1x57hirv8y7p19nl44kqwr-source//./nested-flake1

trace:
subdir for nestedFlake2:
flakeDir for nestedFlake2: /./nested-flake1/./nested-flake2
parentNode.flakeDir for nestedFlake2: /./nested-flake1
outPath for nestedFlake2: /nix/store/s4j8hchzds1x57hirv8y7p19nl44kqwr-source//./nested-flake1//./nested-flake1/./nested-flake2
sourceInfo.outPath for nestedFlake2: /nix/store/s4j8hchzds1x57hirv8y7p19nl44kqwr-source//./nested-flake1//./nested-flake1/./nested-flake2

Notice how nestedFlake2 has /./nested-flake1//./nested-flake1/., repeating nestedFlake1's path.

Context

• Fixes Nested flake source paths broken #13164

• Additional discussion in Support relative path inputs flake-compat#71

cc @edolstra @roberth @packruler @meskill

Testing

I assume a test case would be beneficial, however I'm not familiar with your test suite. Feel free to take over this PR or cherry-pick the changes into a new PR if that is easier. Or if you have ideas for tests, I can try to implement them with some hand-holding 😀

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This is an automatic backport of pull request #13170 done by [Mergify](https://mergify.com).