[cxxmodules] Also preload TMVA/TreePlayer/Graf

ROOT can't autoparse classes inside namespaces with the rootmap
system (as the loading callbacks don't correctly land where
they are supposed to land with our injected namespaces). As this
turns out to be a feature of some kind, let's preload
TMVA/TreePlayer/Graf to fix all failing tests that are related
to this feature/bug with modules enabled.

This commit can be dropped if we solve on of those problems:

1. figure out how to fix this bug in the rootmap-based loading
without regressin in performance.

2. replace the rootmap system with something else like attaching
all C++ modules on startup.

Note that we already do something like this in normal ROOT by
including these packages into the PCH which also makes those
decls available in the normal clang lookup.

Author: Teemperor

CMakeLists.txt

@@ -343,7 +343,11 @@ endif()
 
 if(runtime_cxxmodules)
   # Dummy target that does nothing, we don't need a PCH for modules.
-  add_custom_target(onepcm)
+  # However, we require that TMVA/Graf/Treeplayer are built here as
+  # we will always load them to workaround the fact that we can't
+  # load the decls in them via rootmap files (as they are inside
+  # namespaces which isn't supported).
+  add_custom_target(onepcm DEPENDS TreePlayer TMVA Graf)
 else()
   add_custom_command(OUTPUT etc/allDict.cxx.pch
                     COMMAND ${CMAKE_COMMAND} -E env ROOTIGNOREPREFIX=1 ${PYTHON_EXECUTABLE} ${CMAKE_SOURCE_DIR}/etc/dictpch/makepch.py etc/allDict.cxx.pch ${__allIncludes} -I${CMAKE_BINARY_DIR}/include

core/metacling/src/TCling.cxx

@@ -1372,6 +1372,9 @@ TCling::~TCling()
 void TCling::Initialize()
 {
    fClingCallbacks->Initialize();
+   // Load modules that we can't automatically load via rootmap files as they
+   // contain decls in namespaces which aren't supported.
+   LoadModules({"TMVA", "TreePlayer", "Graf"}, fInterpreter);
 }
 
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