There is little need to expose the guts of this function even within the
library. Make it static in be_local.c, and clean up a few other things
since we know exactly where it is being called from:
* Remove unnecessary origin checks in _cache_get_*() methods- if you are
calling a cache method your package type will be correct.
* Remove sanity checks within local_db_read() itself- packages will
always have a name and version if they get this far, and the package
object will never be NULL either.
The one case calling this from outside the backend was in add.c, where
we forced a full load of a package before we duplicated it. Move this
concern elsewhere and have pkg_dup() always force a full package load
via a new force_load() function on the operations callback struct.