Revert "install:module <value>s must be mutually exclusive"

This reverts commit 768ea2924680fc4beb75a782cb0faf26695fee53, which was
completely misguided---configmgr allows oor:external values to let a "lower"
value shine through when the backend says it cannot produce a value, even when
that "lower" value comes from the same layer (as is e.g. already used in
officecfg/registry/data/org/openoffice/VCL.xcu).

Conflicts:
	postprocess/CustomTarget_registry.mk

Change-Id: Ic8f0c9c931e2c73c9bfb8fbe5a57f60ad6aebde1