Reuse last koji event
While resolving a compose before generating it from a particular koji
tag, if tag and its parents through out its inheritance have not changed
since last koji event, reuse that koji event again instead of call API
to get a new one.
A module dict is used as a simple cache during ODCS execution. It maps
koji event ID from a koji tag.
Signed-off-by: Chenxiong Qi <cqi@redhat.com>