As a step towards a required query for GraphQL implementation in #151 as
well as being a useful check anyway, we can introduce a common report
for listing the usage of a given package, and optionally at a specific
version.

This implements this as both a CLI and web-driven report. To make it
easier to use on the web, we can introduce a form to make it easier to
set the values.

There's a little bit of awkwardness with trying to query `version` and
`current_version` as part of the same query as not being present, where
we end up with an empty set of results. For now we can split this into a
separate query, and follow-up in #324.

This also proves a new model for performing `union`'d queries for
reports, rather than implementing an interface in each datasource.