contributes_to creates configuration conflicts when multiple modules are used

The implementation of extension interfaces with contributes_to explodes with zope.configuration conflicts when an interface and its contributors span multiple webservice:register directives. find_interfaces_and_contributors is run once per webservice:register, and does all of the aggregation internally, not considering that there might be other modules involved.

This is a fatal flaw for the situation that contributes_to was meant to solve: it's not possible to have entry extensions pluggable by optional bits of ZCML.