FIX part of #1959: declaration versions were not being incremented correctly
We were building a mapping from ModuleName to [Occ] from the usage list, using the usg_mod field as the key. Unfortunately, due to a very poor naming decision, usg_mod is actually the module version, not the ModuleName. usg_name is the ModuleName. Since Version is also an instance of Uniquable, there was no type error: all that happened was lookups in the map never succeeded. I shall rename the fields of Usage in a separate patch. This doesn't completely fix #1959, but it gets part of the way there. I have to take partial blame as the person who wrote this fragment of code in late 2006 (patch "Interface file optimisation and removal of nameParent").