xAODTruthCnvAlg update

This MR is a follow up on the discussion in the core SW weekly meeting on Sep 17, 2020.

Two changes have been made in xAODTruthCnvAlg:

1. Use EventInfo instead of EventStreamInfo for retrieving mcChannelNumber
2. Access TagInfo information from the input metadata store in the first handler of BeginRun incident. Cache this information and use it for setting values of relevant fields of Truth metadata

Also dropped atlas_depends_on_subdirs() from CMakeLists.txt

CC: @zmarshal