Ignore hidden directories in workspace discovery (#5408)
## Summary
This is surprisingly complex because we need to decide what happens if
you run `uv run` from within a hidden folder, etc. For now, I did the
simplest thing: we just ignore workspace members that are hidden
directories if they lack a `pyproject.toml`, so you can still include
hidden members, they're just ignored if they don't seem to be projects.
Closes https://github.com/astral-sh/uv/issues/5403.
