dependencies api: Implement `Dependents`

Created by: efritz

After #35435 (closed) is implemented, we can write a Dependents method that...

• queries data from codeintel_lockfiles/codeintel_lockfile_references (see query below)
• queries data from lsif_packages/lsif_references

...and merges the two results together.

The query that will need to be written to get inverse lockfile data will be something like (shorthanded SQL):

select repository_id, commit_bytea
from codeintel_lockfiles
where codeintel_lockfile_reference_ids @> (
    select array_agg(id) -- or some other crazy incantation
    from codeintel_lockfile_references
    where repository_id = %s and commit_bytea = %s
)

The query needed for precise code intel is described in this comment.