main added hermes_cli/left_core_migration.py, imported by source_build.py; the
R15 import-graph test caught it unrouted on the composite of all seven PRs.
(cherry picked from commit 338aabcdda316a92b8bfcae10f52730fd6fcb344)
(cherry picked from commit 5a839bb475d08f82293f07bcdb60ded859814476)