Ensures 3rd party jars on path prior to build; builds egg, not egg-info

If required third party jars are not already on sys.path, makes it so. More interestingly, works around some convoluted logic in setuptools that determines whether an egg should be built, vs egg-info. We want pth support, hence the need for egg. Read the comment for a bit more on how this logic is actually implemented; the companion frame inspection logic in setuptools is here:
https://bitbucket.org/pypa/setuptools/src/64078530af019b31b3feae5cab73bf5c48ce4898/setuptools/command/install.py?at=default#cl-64