Detect direct dependents missing from release (#193)

If the user includes a package in the release and selects a new major
version, currently, we display a message that requires the user to
include all peer dependents in the release. However, since we moved peer
dependencies to dependencies in the `core` repo, realistically, this
error will never appear, and instead we want to strongly advise the user
to include all *direct* dependents in the release.

To do this we have to modify the core logic to detect missing direct
dependents separately from peer dependents.

Author: mcmire