Ignore invalid extras from PyPI (#1731)
## Summary
We don't control these, so it seems preferable _not_ to fail on them,
but rather, to just ignore them entirely. (I considered adding a long
allow-list, but then questioned the point of it? We'd end up having to
extend it if more invalid extras were published in the future.)
Closes https://github.com/astral-sh/uv/issues/1633.
