Ensure relative path inputs are relative to the parent node's _actual_
`outPath`, instead of the subtly different `sourceInfo.outPath`.
Additionally, non-flake inputs now also have a `sourceInfo` attribute.
This fixes the relationship between `self.outPath` and
`self.sourceInfo.outPath` in some edge cases.
Fixes#13164
This requires that we refer to the `sourceInfo` instead of the
`result`. However, `sourceInfo` does not create a chain of basedir
resolution, so we add that back with `flakeDir`.
