Remove indirection on get_name and get_version operations

Remove indirection on get_name and get_version operations

For a package to be loaded from any of our backends, these two fields
are always required upfront. Due to this fact, we don't need them to be
backend-specific operations and can just refer to the field directly.

Additionally, our static (and thus private) cache package accessors had
a NULL check on pkg before returning the relevant field. Eliminate this
since they only way they are ever called is via the packages attached
callback struct, which would have caused the NULL pointer dereference in
the first place.

Signed-off-by: Dan McGee <dan@archlinux.org>