This patch adds a new config option. This is expected to be a name of
subdirectory in the repo with module defaults. If supplied, overrides
from that location are loaded every time defaults are loaded.
This raises the minimal required version of libmodulemd to 2.8.0
JIRA: COMPOSE-3828
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>