Fix broken URLs parsed from relative paths in registries (#1413)
Closes https://github.com/astral-sh/uv/issues/1388
Fixes incorrect handling of relative paths returned by indexes without
an explicit `<base>`.
`Url.join` will drop the last segment in an url e.g. `http://foo/bar` ->
`http://foo/baz` if there is not a trailing slash but what we want is
`http://foo/bar/baz`. We don't add the trailing `/` in
`base_url_join_relative` because flat indexes are `http://foo/bar.html`
and we _want_ `bar.html` to be replaced.
