dependencies: Proactively parse lockfiles in background

Created by: efritz

Fixes #31643 (closed).

Test plan

• Existing tests in CI and newly added unit tests
• Manual e2e testing:
  • Add "codeIntelLockfileIndexing.enabled": true to site config
  • Create global auto-indexing policy
  • Wait and then confirm with psql

  sourcegraph@localhost:5432 sourcegraph# select * from last_lockfile_scan;
   repository_id |     last_lockfile_scan_at
  ---------------+-------------------------------
               1 | 2022-05-23 12:59:50.152192+02
               2 | 2022-05-23 12:59:50.152192+02
               3 | 2022-05-23 12:59:50.152192+02
  (3 rows)
  
  sourcegraph@localhost:5432 sourcegraph# select * from codeintel_lockfiles;
   id | repository_id |                commit_bytea                | codeintel_lockfile_reference_ids
  ----+---------------+--------------------------------------------+----------------------------------
    1 |             1 | \xf144a35ed4ac538fae93fa3783175108738f71b9 | {}
    2 |             2 | \xbaf6e34a292ff7b57184d2987d79f0352987d8b4 | {1,2}
    3 |             3 | \x285adcc5ced0bb267a7c874cfa3ca238266ce14f | {3,4,5,6,7}
  (3 rows)