fix: importActual path resolution for non-child paths (fix #1864)

[simon-abbott]

This fixes two similar bugs with importActual's path resolution:

1. #1864, importing from sibling directories would try to import from sub directories instead.
2. Importing from a sibling directory that starts with the root path would snip the root off the path (for example, path = '/foo/bar-baz' with root = '/foo/bar' results in an output of -baz)

Fixes #1864.

[simon-abbott]

Ok I have no idea why that one CI run failed. If anyone knows please let me know so I can fix it!

[simon-abbott]

@antfu Could this be merged soon? This is completely blocking my ability to convert our application over to vitest.

[sheremet-va]

@simon-abbott We might try this approach anyway. Do you think you can resolve conflicts?

[simon-abbott]

Certainly!