find_package: Add support for default GLOBAL imported targets

Allow find package to promote scope of imported targets by specifying
an argument to `find_package` or by specifying a CMake variable.
    * Add support for CMAKE_GLOBAL_IMPORT_SCOPE variable
    * Add support for GLOBAL argument to find_package

Additionally add testing for above features.

Source/cmAddExecutableCommand.cxx

@@ -54,6 +54,10 @@ bool cmAddExecutableCommand(std::vector<std::string> const& args,
     }
   }
 
+  if (importTarget && !importGlobal) {
+    importGlobal = mf.IsImportedTargetGlobalScope();
+  }
+
   bool nameOk = cmGeneratorExpression::IsValidTargetName(exename) &&
     !cmGlobalGenerator::IsReservedTarget(exename);