ST6RI-909 Circular recursive import causes infinite recursion in getImportedMembership

[seidewitz]

This PR corrects a bug that caused an infinite recursion in the computation of imported memberships in a model with a circular recursive import. For example, with the model

package Test {
    package P {
        public import Test::**;
    }
}

calling getImportedMembership on the element Test::P resulted in a StackOverflowError.

The bug was corrected by updating MembershipImportAdapter::importMemberships to properly check whether the importedElement was in the excluded set before recursing. (Note that a similar check was already proper implemented in NamspaceImportAdapter::importMemberships.)