add zipapp support with bundled dependencies

[gaborbernat]

Will resolve #1464.

Creates a zipapp that has all dependencies (for all platforms), using the python2.7 dependency versions and validate these provide the same functionality as one would install into a python.

dependencies:
appdirs-1.4.3-py2.py3-none-any.whl
importlib_metadata-1.4.0-py2.py3-none-any.whl
importlib_resources-1.0.2-py2.py3-none-any.whl
more_itertools-5.0.0-py2-none-any.whl
pathlib2-2.3.5-py2.py3-none-any.whl
six-1.13.0-py2.py3-none-any.whl
zipp-0.6.0-py2.py3-none-any.whl
distlib-0.3.0-py3-none-any.whl
virtualenv-0.1.dev39+gf3036b8.d20200113-py2.py3-none-any.whl
zipapp created at virtualenv.pyz

Maybe we should pin these version 🤔 Down the line we might have some dynamic way of selecting the correct library version, but for now, 2.7 versions handles all 3 too...

[gaborbernat]

Finally, that works on all platforms!