repoman/main/py: Create new scan.py and scan()

This moves the main scan code out to a function.
Comment out some unused variables.
repoman/main.pt: Move some additional code to repos.py
repoman/main.py: Create a new XmlLint class

Create the new class in _xml.py.
Consolodate all the xmllint code in this one class.
repoman/main.py: Create a new Changes class

This new class is to scan for and hold the changes in the repo.
Later it will act as a manager class to run the individual VCS plugin modules scan function.