Implement the charm-related API.

Also move MgoTestPackage suite registration out from api_test to its
own package_test.go.
For each iface we need to retrieve all the entities providing (or requiring) that iface, and include their id in the result.
The underlying decision here is that, rather than executing one query for each iface, we retrieve all the entities in a single query. After that we need to group the ids (and optionally meta) of those entities by iface programmatically. That's what getRelatedIfaceResponses does.