Move expand action constants to Dependable module

PRUNE, SKIP, and KEEP_BUT_PRUNE_RECURSIVE_DEPS were defined
separately on Dependency and Requirement with identical values.
This coupling was fragile — if either constant changed, the
polymorphic code in dependencies_helpers.rb would silently break.

Move the constants to the shared Dependable module (included by
both classes) so there is a single source of truth, and update
all callers to reference Dependable:: directly.