Fix issue in aptpkg and yumpkg modules related to holding/unholding packages

Fixing issues where packages were being put on hold even when test=True.  Fixing issues related to listing and putting packages on hold using yumpkg, certain assumptions about the results of yum-versionlock were made which were false.
Adding some checks to ensure a package exists in the available repositories if not passed with a source URL, before allowing to be put on hold.