Rollup merge of #139647 - eholk:package-namespace, r=fmease

Add unstable parsing of `--extern foo::bar=libbar.rlib` command line options

This is a tiny step towards implementing the rustc side of support for implementing packages as optional namespaces (#122349). We add support for parsing command line options like `--extern foo::bar=libbar.rlib` when the `-Z namespaced-crates` option is present.

We don't do anything further with them. The next step is to plumb this down to the name resolver.

This PR also generally refactors the extern argument parsing code and adds some unit tests to make it clear what forms should be accepted with and without the flag.

cc ```@epage``` ```@ehuss```