As a means to allow us to centrally control the way that we open the
database, as well as allowing us to modify the querystring in a central
place in the future, or apply further checks.

In most cases, this is where a report command is created, the query is
set up but there's no implementation in a datasource, which is a bug.

To make it easier to implement a new query with a datasource and
reduce the work to wire it all in, we can introduce a `querier` and
`query...` interface that can be implemented by a datasource.

This also moves the `Name` method to the `querier` interface as it's
only used here.

As a fixup for 363bed0e and #37.

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