Rascal rename: detect ambiguous/double declarations at use sites properly

[toinehartman]

Describe the bug

Currently, this test fails:

@expected{illegalRename}
test bool multiModuleAmbiguous() = testRenameOccurrences({
    byText("Foo", "public int foo = 1;", {0}),
    byText("Bar", "public int bar = 2;", {}),
    byText("Main", "import Foo;
                   'import Bar;
                   'int baz = foo + bar;", {0})
});

Expected behavior
It should succeed (a.k.a. throw illegal rename errors).