create_intermediate_modules should take pkg origin into account

[edublancas]

I just realized that create_intermediate_modules creates the intermediate modules in the current directory, but this isn't always the case.

If the user modified the PYTHONPATH, or installed a package with pip install --editable ., the pkg may be in a different location.

To solve this, we can use find_spec, and pass the first element. e.g., if source_parts is a.b.c, use find_spec('a').origin to get the location of module a, and use this a prefix to create the intermediate files. If a, does not exist, then it's fine to create it in the current working directory

[edublancas]

cc @fferegrino