Fix AttributeError on postcommit when db-create adds non-system CWSource entities
In this case, `Repository.add_source` is called from an operation, not from
`Repository.init_sources_from_database`, thus the attribute `sources_by_eid`
does not exist.
Closes #16251078.
[devctl] Optimize cleanup_sys_modules a bit
by moving out of a loop a costly computation that is constant.
This at least benefits the `i18ncube` command.
Related to #15789486.