[cxxmodules] Don't include existing headers again in rootcling.
[cxxmodules] Don't include existing headers again in rootcling.

Some tests are failing with modules such with redefinition errors
(for example `roottest-cling-parsing-scopeDict-build`).

The reason for this is that we include all headers that form
the C++ module again to make sure the module is complete (some
rootcling invocations don't have all headers of the module as
arguments, most notably the Core module). The first inclusion
of these headers happens from the normal rootcling code.

This patch changes this behavior that we only include the
*missing* headers that are necessary to make the C++ module
complete and not all headers.

karies/root,karies/root,olifre/root,olifre/root,karies/root,olifre/root,olifre/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,karies/root,root-mirror/root,olifre/root,olifre/root,olifre/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,karies/root,karies/root