[3.6] bpo-30645: don't append to an inner loop path in imp.load_package() (GH-2268...

[3.6] bpo-30645: don't append to an inner loop path in imp.load_package() (GH-2268) (#2364)

Bug didn't manifest itself when importing a module with source as .py files are always the first on the search path. The issue only showed up in bytecode-only packages where the calculated file path would be ``__init__.py/__init__.pyc``.

Patch by Alexandru Ardelean.

(cherry picked from commit c38e32a10061a7c6d54e7e53ffabf7af7998f045)