Ensure all dependencies are declared

[elijah.epifanov]

Package breakage prevention idea:

ldd all executable files and check that all their dependencies belong to packages that are direct/transitive dependencies (paludis --owner '/usr/lib/lib.....so' is in recursive dependencies tree). Some packages build against some dependency that isn't declared and when this dependency will be removed (i.e. I temporary removed libpq, because new postgresql had problems building when old libpq was present), but they will fail at runtime.

[ciaranm]

see ticket:129