new-build stores are dbs in the ghc-pkg sense, but treating them as such breaks cabal

e.g.

1) the user `ghc-pkg unregister`s a dependency of a build-tool
2) the user deletes the corresponding package
3) cabal new-build will choke with some internal error when creating a build-plan involving that build-tool because it expects the dependencies of build-tools to be installed but they are not.

Also note that after 1+2, `ghc-pkg check` turns up clean, while from cabal perspective, the store is in an inconsistent state. Well, that depends on the "consistency" definition, but alternatively, the store still _is_ consistent, only the build-plan construction makes false assumptions which would make it a bug there instead.

Possible solutions:

1) Properly communicate to users that the store is not a proper package-db. Ideally make it incompatible with `ghc-pkg`
2) Fix the build-plan to be more robust against this, e.g. by not expecting dependencies of executables to be installed.