Analyze Scripts not in a Package

Currently we only analyze scripts that are part of a setup.py described package. This PR adds extra code to the scanner that will bundle up all unaccounted scripts, and put them in a source unit of their own.
Another Python case that's not handled right now are web apps that don't have a setup.py (because they're never meant to be distributed or depended on in anyway). These typically still have a requirements.txt to describe their own dependencies. It seems like we should handle this the same way as this PR handles scripts (i.e., throw everything into one source unit and go from there). Maybe "PythonApplication" or "PythonProgram" would be a more general source unit name than "PythonScripts"?