Avoid trailing slash when deserializing from lockfile (#9848)
## Summary
Very tricky problem whereby `workspace_root.join(path)` returns the
workspace root with a trailing slash if `path` is empty... This caused
us to accidentally _include_ excluded members during workspace
discovery, since (e.g.) `packages/seeds` doesn't match
`packages/seeds/`.
Closes
https://github.com/astral-sh/uv/issues/9832#issuecomment-2539121761.
