package comparison : removal of d5->d6 upgrade path means D7 package schema version not detectable

This is a rather subtle bug that won't cause any data loss, and in the current code is only being displayed as an error but isn't actually an error.

the example here is book.module , which had an update_6000, but that function is no longer present in the d7 book.install file, because it was only used in the d5 -> d6 upgrade process.