Fix scope of transitive target name lookups

In cmTarget, cmGeneratorTarget, and cmGeneratorExpressionEvaluator, fix
target name lookups to occur in the cmMakefile context of the target
that referenced the name, not the current 'head' target.  The context
matters for imported targets because they are directory-scoped instead
of globally unique.  We already do this in cmComputeLinkDepends and
cmComputeTargetDepends.

Extend the InterfaceLibrary test with an example covering this behavior.

Tests/InterfaceLibrary/definetestexe.cpp

@@ -15,7 +15,9 @@
 #error Expected IFACE_HEADER_BUILDDIR
 #endif
 
+extern int sub();
+
 int main(int,char**)
 {
-  return 0;
+  return sub();
 }