Also, make sure CPyCppyy headers are in the PCH.

This commit contributes to fix the roottest-python-cling tests,
which use TPython, now part of CPyCppyy.

Can cause unnecessary extra linking to system libraries. We should
add linking libraries directly to each target that needs it.

Not expose ROOT/RConfig.h to rootcling, but to add the header to
ROOT_Config pcm.

This patch includes:
- Remove existing modulemap.overlay files
  Cling now can generate virtual overlay files on-demand.
- Implement `-modulemap_overlay` flag to Cling
  This flag is used to tell Cling the location of modulemaps to be
  overlayed. (eg. stl.modulemap, libc.modulemap)
- Generate and load virtual modulemap in Cling
  It is in Interpreter constructor, happens as part of cxxmodules
  initialization step.
- Implement an interface to Clang CompilerInvocation which can take FileSystem pointer
  Previously, Clang only took a "string of filenames" which clearly doesn't
  work for our usecase. We already discussed this new interface at
  modules meeting, so this change will land upstream.
- Add a file existence check in Clang
  When compiling a pcm and when Clang saw the #include with which pcm was available
  (or being generated implicitly), Clang was putting a notation of the full path to this pcm.
  This caused an error when build directory was deleted, because the path didn't exist anymore.

This patch enables modules to be binary distributed, and to make it
work in CMSSW enviroment.

On UNIX, build/unix/compiledata.sh is used, while on
Windows, cmake/scripts/compiledata.win32.in is used.

The aim of this patch is to make runtime modules "installable".

Previously, modulemap.overlay.yaml file was being generated by cmake
__only__ for build directory. So the path/to/your/build/directory was
hardcoded to modulemap.overlay.yaml and Cling was reading modulemap for
stl and libc from the build directory even if it was installed to different
location.

This was a problem for CMSSW, so in this patch I'm generating two
overlay files, one for build directory and other one for installed
directory. CMake will generate modulemap.overlay.yaml for __build__
directory from modulemap.overlay.yaml.in.build at compilation time, and
it will generate and install modulemap.overlay.yaml for __install__
directory from modulemap.overlay.yaml.install.

The script git_coreteam.py uses outdated gitstats information
from the web, while the new script uses the git shortlog from the
repository itself (with a little help of recently added .mailmap).

Instead of removing the *.pyc file in argparse2help.py,  we avoid to generate such files
by invoking argparse2help.py with option -B.

- remove trailing white spaces.

Implement an "argparse" unique description of man pages and usage for root commands. 

This error: http://cdash.cern.ch/viewBuildError.php?buildid=567476
was caused because modules couldn't merge definition of struct __pthread_internal_list

Previously, [system] was used to mark modules which were supporsed be
implicitly built by rootcling for runtime modules. This was a workaround, and it was causing
SourceLocation error in cxxmodules as it is also using the same
modulemap.

To remove the workaround, we are introducing a new convention that modules
start with ROOT_ is treated as "ROOT system modules", which can be build
implicitly.

This reverts commit dc825fc4.

* Centralise the generation of help for root command

Add argparse2help.py, add root-argparse.py, delete the old help of root, add the cmake commands needed to generate the header containing the command line options help and to generate manual page for the root command.

The python file argparse2help.py can be called with the root-argparse.py and the output name as arguments. This call will generate the header file if the output has an extension ".h" and will generate the manual page if the extension is ".1"

* Centralise the generation of help for the rest of the commands

Add cmake changes to generate headers and manual pages for rootcling hadd and hist2workspace
Delete old man pages of hadd and hist2workspace
Add commented out generation of man pages for python commands, we need to add a dependency before uncommenting them out
Add hadd rootcling hist2workspace argparse modules

* make the path compatible with windows

* review modifications

* add dependency on the input and the generation script of man pages

* remove installation of headers

* Update CMakeLists.txt

It makes no sense to generates man pages on Windows. Windows does not have the `man` command.

* Update argparse2help.py

* Update argparse2help.py

* Update argparse2help.py

* Update hadd.cxx

haddCommandLineOptionsHelp.h was not included for windows

* Update root-argparse.py

add the option --version

* Update argparse2help.py

simplified version (Thanks Bertrand !)

in ROOT6 there is only one kind of reflection information. For this
reason, also the neighbouring references to Reflex have been removed.

this is done until the present ABI issues between gcc and clang are
solved, most notably the ones concerning shared_ptrs.