LSIF: RFC127-4: Some light API cleanup

Created by: efritz

This PR handles some misc cleanup, notably it changes the API introduced in https://github.com/sourcegraph/sourcegraph/pull/9179 so that the dump manager is not returning entire document data payloads (which can be sizeable). Instead it returns monikers attached to a range (which is what the server was using the document for in the first place), or returns package information by ID.

This also takes the opportunity to remove some code that was there to support leaky integration test that no longer exist.