Disallow porcelain to populate includes and defines of IMPORTED targets.

With similar reasoning to the parent commit, as downstreams, we can't
determine what $<CONFIG> generator expressions would be appropriate.

Upstream would have populated the INTERFACE_INCLUDE_DIRECTORIES with
config-specific generator expressions, possibly appropriate for
their DEBUG_CONFIGURATIONS. In theory, if we would add include
directories for a DEBUG intent, we would have to match the upstream
configurations for that.

Rather than attempting to discover the appropriate configurations
at this time, simplify the feature instead. The use of IMPORTED targets
with these commands could still be added in the future if targets
would export their DEBUG_CONFIGURATIONS somehow.

Tests/CMakeCommands/target_include_directories/consumer.cpp

@@ -2,7 +2,6 @@
 #include "common.h"
 #include "publicinclude.h"
 #include "interfaceinclude.h"
-#include "importedinterfaceinclude.h"
 
 #ifdef PRIVATEINCLUDE_DEFINE
 #error Unexpected PRIVATEINCLUDE_DEFINE
@@ -16,10 +15,6 @@
 #error Expected INTERFACEINCLUDE_DEFINE
 #endif
 
-#ifndef IMPORTEDINTERFACEINCLUDE_DEFINE
-#error Expected IMPORTEDINTERFACEINCLUDE_DEFINE
-#endif
-
 #ifndef CURE_DEFINE
 #error Expected CURE_DEFINE
 #endif