As noted in #73, we've got a few cases where we're fetching the whole
set of dependencies we've got across the database, but only using a
subset of the data, which can be duplicated between other rows.

We can replace this by doing a `SELECT DISTINCT` query for only the
fields that are necessary.

Unfortunately we cannot change this for `endoflifedate.Generate` (as
highlighted in #80).

For a - larger - dataset I'm testing against, this reduces from ~73k to
~16k rows being checked, which significantly improves performance,
especially taking into account the way these are then used for HTTP
calls.

For now, we can directly depend on the
`renovatedb.RetrieveDistinctPackagesRow` struct, but would likely want
to break this dependency by adding a layer of abstraction in the future.

Closes #73.