Repoman: Refactor VCSStatus to pass non consistent data to the check()

This will facilitae for the initialization of the class before the big xpkg loop.
Repoman: Refactor PkgMetadata and XmlLint classes for variable data passed in

Move all non-consistent data to be passed in via the check functions.
Initialize XmlLint once in the PkgMetadata class __init__().
repoman/main.py: Move the check class instantiations out of the xpkg loop

This should help reduce overall run time when doing large repoman full runs.
It instead re-uses the class instances for each pkg checked.