Simplify par Launcher

Instead of copying some of Py_Main and changing it, use Py_Main
directly, paired with a __main__.py script that uses the runpy module to
trampoline into the user code.

Test: build/soong/python/tests/runtest.sh
Change-Id: I7f5e1e38b63b4e2ce83a85c08953a5e67c7b8788