Every implementation in documents.py and rebuild.py takes the session it
writes to, so a set cannot reach one: the facade narrows once and passes the
database on, rather than checking and carrying a union. Tests calling an
implementation directly go through `writing()`.
haiku.rag.ingester.sources was never ingester-only: one-shot client
ingestion resolves adapters through it (create_document_from_source), and
convert() now fetches through HTTPSource, so the core client imported into
the ingester package to reach them.
Move the package to haiku.rag.sources and update every import. No shims:
haiku.rag.ingester.sources is gone.
The haiku.rag.sources plugin entry-point group is unchanged, so third-party
source packages need no edit — the group name now matches the module path it
always implied.
Source unit tests move to tests/sources/. test_source_plugins.py stays in
tests/ingester/: it drives a PeriodicPoller against the job repo, so it is
plugin wiring through ingester machinery rather than a source test.
