paths.relativize doesn't support a path not fully under start

[thomasvl]

The docs for paths.relativize compares itself to os.path.relpath, but the current implementation requires path be fully under start.

In theory it could also work for things that have a common prefix:

• paths.relativize("foo/bar/baz", "foo/mumble") could return "../bar/baz"
• paths.relativize("foo/bar", "foo/mumble/baz") could return "../../bar"
• paths.relativize("foo/bar/baz", "mumble/grumble") could return "../../foo/bar/baz"

Right now these cases fail, so adding the support shouldn't break anyone. If callers want something fully nested, they could check the result to ensure it doesn't start with ...

[jfirebaugh]

Aspect's Bazel library has this: https://docs.aspect.build/rules/aspect_bazel_lib/docs/paths/#relative_file