packages: fix package ref listing query with limit

[Strum355]

The list package repo refs query accidentally worked under the assumption of what the data should ideally look like (unique (scheme,name) tuples). This is not true in stage 1 of the migration, where there is still one (scheme,name) tuple for every version of that tuple.
Continuing: in step 1 of the migration, we copy for every row in lsif_dependency_repos into package_repo_versions, using the smallest ID for every (scheme,name) as the "canonical" package repo entry ID (this is so when we collapse n into 1, every entry in package_repo_versions still points to an existing package repo reference).
Thus when joining the two tables, every entry besides the first for every (scheme,name) would not have an accompanying entry in the latter table, thus not being in the final result set. This happens after the limit factor, so we would end up with <= limit.

There was also a bug with the count section that was fixed here, because the reduce happens in-application instead of in SQL :^)

Test plan

Added extra part in existing unit test and tested the extra part with before and after

[efritz]

I tend to use 'rank' for row number (not a bit, just a fun fact)