As part of #7, we want to provide common queries that can be used by
consumers of `dmd`, to reduce their need to hand-roll some common queries.

To start with, we can provide a query to list number of packages per
package manager.

This introduces a pattern for doing so, by:

- adding a new query to datasources that support it
- add a common data type in the `queries` package
- add a `report` subcommand for querying
- use `go-pretty` to format as a table