cmTarget: Allow transitive evaluation of SOURCES property.

Extend the cmGeneratorExpressionDAGChecker with an interface
returning the name of the top target.  Use that to determine
when there is a DAG violation, as required by the RunCMake.Languages
tests.

Tests/ConfigSources/iface_src.cpp

@@ -0,0 +1,5 @@
+
+int iface_src()
+{
+  return 0;
+}