Augment `--verifytypes` to report missing stubs when native extension modules are present

If a library includes a native extension module (e.g. a ".so" file on Mac) but has no corresponding type stub, it would be nice if `--verifytypes` included this in its type completeness report.

See [this discussion](https://github.com/microsoft/pyright/discussions/10615#discussioncomment-13487596) for details.

Start with the --verifytypes entry point and read the linked discussion for the expected treatment of native extension modules. Trace how the type completeness report discovers package modules, then identify the relevant tests before implementing coverage for an extension with no corresponding stub. Done means the report identifies that missing stub consistently.

Written by the indexing model from the issue text.