Require disambiguated relative paths for `--index` (#14152)
We do not currently support passing index names to `--index` for
installing packages. However, we do accept relative paths that can look
like index names. This PR adds the requirement that `--index` values
must be disambiguated with a prefix (`./` or `../` on Unix and Windows
or `.\\` or `..\\` on Windows). For now, if an ambiguous value is
provided, uv will warn that this will not be supported in the future.
Currently, if you provide an index name like `--index test` when there
is no `test` directory, uv will error with a `Directory not found...`
error. That's not very informative if you thought index names were
supported. The new warning makes the context clearer.
Closes #13921
