bake show does not display required dependencies of submodules

This is a move of bugzilla issue 1947: https://www.nsnam.org/bugzilla/show_bug.cgi?id=1947

Configure 'ns-3-allinone' on a system without 'automake' installed. './bake.py show' will not list the dependency, but './bake.py build' will produce a build error because click-dev cannot build without automake.

The issue is that optional submodules that have hard dependencies do not show up in './bake.py show'; only the dependencies of the main module and required submodules.

This manifests itself with click routing which depends on automake.