Fix an issue importing stages from entrypoints.
Fix an issue importing stages from entrypoints.

If the stage module itself calls "import streamcorpus_pipeline" for
whatever reason (maybe indirectly), then we can't try to load stages via
entry points when streamcorpus_pipeline gets imported.  This was happening
via the yakonfig sub_modules declaration; instead populate that after
import time in replace_modules() (where we already searched for external
modules from other sources).

trec-kba/streamcorpus-pipeline,trec-kba/streamcorpus-pipeline,trec-kba/streamcorpus-pipeline