cxxmodules: rework control logic for scanning regular C++ sources

Now that scanning support is no longer experimental, the logic for
whether or not to scan C++ 20 sources is now important because all
projects are now exposted to the logic. Make the scanning rules explicit
in the documentation and rework the queries to localize all of the
associated logic.

A policy to handle the ultimate fallback logic will be implemented in a
following commit.

Source/cmVisualStudio10TargetGenerator.cxx

@@ -362,15 +362,6 @@ void cmVisualStudio10TargetGenerator::Generate()
     this->GeneratorTarget->CheckCxxModuleStatus(config);
   }
 
-  if (this->GeneratorTarget->HaveCxx20ModuleSources() &&
-      !this->GlobalGenerator->SupportsCxxModuleDyndep()) {
-    this->Makefile->IssueMessage(
-      MessageType::FATAL_ERROR,
-      cmStrCat("The target named \"", this->GeneratorTarget->GetName(),
-               "\" contains C++ sources that export modules which is not "
-               "supported by the generator"));
-  }
-
   this->ProjectType = computeProjectType(this->GeneratorTarget);
   this->Managed = this->ProjectType == VsProjectType::csproj;
   const std::string ProjectFileExtension =