[mypyc] Set dunder attrs when adding module to sys.modules

[p-sawicki]

Recently there was a change to add native modules to sys.modules before they are executed to be able to detect circular imports. This introduced a regression when the module is a package that imports objects from other files within the package, eg. from pkg.file import something inside pkg/__init__.py. Such imports result in an exception ModuleNotFoundError: No module named 'pkg.file'; 'pkg' is not a package., for example when trying to upgrade mypy in black.

This error is raised because Python expects the parent module of file to have the __path__ attribute set when resolving the import but we don't set this attribute before adding the pkg module to sys.modules.

So use existing functions to set relevant dunder attributes (__path__ for packages and __file__, __spec__, and __package__ for all) before registering the module in sys.modules.

Don't skip compilation for __init__.py files in separate compilation mode to make this possible to test.

Use Py_CLEAR instead of Py_DECREF on the import object on failure in CPyImport_ImportNative as the import object might be freed when deleting it from sys.modules. This triggered an assertion when running tests with a debug build of cpython.