Fix capabilities files generation

The generation of capabilities files was added.
CMS indeed uses those to feed the FW plugin manager, which is
very similar to the good Old SEAL plugin manager.
DGRs can have a decl to be emitted, another not to be emitted.

When this oce comes from a #include through ParseForModules():
class A{} g;
A should be emitted, but not g.
This is now tested in roottest/root/meta/templateAutoload.