[arch-general] Implement sql/sqlite database for pacman local database

[Allan McRae]

As the currently lead pacman developer...   We will never have a sql (or
other) database backend.

When we did tests for the sync backends, using a single tar file gave
the same speed-up as using some sql variant (and we still have not
optimised any reading from that - for the sync "dbs", we always load all
information regardless of what proportion is needed)  I intend to move
the local db to a single tar file too at some stage.

Allan