Namespaces: final work to hierarchize cxxbridge internal mod

After #76 we will still have a bit more namespace work to do. The final output structure should be correct, but within our internal `cxxbridge` mod we'll still have a flat structure. That means we won't allow input where there are identically-named symbols in different namespaces.

To finally fix that, we need to do the `mod` parts of https://github.com/dtolnay/cxx/issues/353 and then reflect in autocxx.

Start by reading the `mod` portions of issue #353 and the namespace changes from #76. Trace how autocxx generates its internal `cxxbridge` module and verify that nested namespaces are represented there, including identically named symbols from different namespaces. Done means the internal structure is hierarchical and the requested namespace inputs are accepted.

Written by the indexing model from the issue text.