Include ABI hash of dependencies in diagram hashes

[byorgey]

Currently, diagrams are not rebuilt when packages they depend on change. A simple solution is to get ghc-pkg to dump out the ABI hash of dependent packages and include them in the hash we compute for each diagram. Some notes:

• Have to figure out how to map from module names to packages.
• Be careful to use the right package database, e.g. check if some hsenv (or cabal sandbox?) is active.