versioninfo: lookup packages case insensitive, like buildout does.
versioninfo: Removed unused imports.
versioninfo: PEP8.
versioninfo: Removed obsolete warning about not supporting git packages.
Added new checkpinnings command.

Lists all Plone / Zope packages whose version pin is overriden in one of our KGSs.
checkpinnings: Also consider versions.cfg one of our "KGSs".
checkpinnings: Minor formatting fix for better readability.
Merge back changes from internal fork.