Add runtime hook to create PyQt5.Qt 'Composite Module'

PyQt5 exposes the PyQt5.Qt module which contains symbols from all other PyQt5
packages. PyQt5 uses Python's C API to import all modules and merge their
contents with PyQt5.Qt (see the generated sipQtcmodule.c file when compiling
PyQt5), but due to a bug in PyQt, it seems to skip all import-hooks machinery.

Here, we replicate what PyQt does but using native Python code, which we know
won't bypass the import hooks mechanism described in PEP-302.