Support pep3149, latest import mechanics, fully

Support pep3149, latest import mechanics, fully

Reworked "sourceless" system to be fully capable of handling any
combination of: Python2/3x, pep3149 or not, PYTHONOPTIMIZE or not,
for locating and loading both env.py files as well as versioning files.
This includes: locating files inside of ``__pycache__`` as well as listing
out version files that might be only in ``versions/__pycache__``, deduplicating
version files that may be in ``versions/__pycache__`` and ``versions/``
at the same time, correctly looking for .pyc or .pyo files based on
if pep488 is present or not. The latest Python3x deprecation warnings
involving importlib are also corrected.

Change-Id: I2495e793c81846d3f05620dbececb18973dd8a8f
Fixes: #449