Collect all modules contained in self

Property definitions

nitc :: model_collect $ MGroup :: collect_all_mmodules
	# Collect all modules contained in `self`
	fun collect_all_mmodules(filter: nullable ModelFilter): HashSet[MModule] do
		var res = new HashSet[MModule]
		for mgroup in collect_mgroups(filter) do
			res.add_all mgroup.collect_all_mmodules(filter)
		end
		return res
	end
src/model/model_collect.nit:456,2--463,4