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.

Source/cmTargetIncludeDirectoriesCommand.h

@@ -60,7 +60,7 @@ public:
       "Specify include directories or targets to use when compiling a given "
       "target.  "
       "The named <target> must have been created by a command such as "
-      "add_executable or add_library.\n"
+      "add_executable or add_library and must not be an IMPORTED target.\n"
       "If BEFORE is specified, the content will be prepended to the property "
       "instead of being appended.\n"
       "The INTERFACE, PUBLIC and PRIVATE keywords are required to specify "
@@ -82,8 +82,7 @@ public:
   cmTypeMacro(cmTargetIncludeDirectoriesCommand, cmCommand);
 
 private:
-  virtual void HandleImportedTargetInvalidScope(const std::string &tgt,
-                                   const std::string &scope);
+  virtual void HandleImportedTarget(const std::string &tgt);
   virtual void HandleMissingTarget(const std::string &name);
 
   virtual bool HandleNonTargetArg(std::string &content,