Refactor to define REALMODULES incrementally without overwriting
'NO_PSEUDOMODULES'.
This also allows an external makefile to maybe add something there if it really
needs to.
Factorize the reused value in a private variable.
I define it private as somehow 'USEPKG' is supposed to be removed so the
variables can be removed later.
BASELIBS is used only in the main Makefile.include or included files.
As it is not used in sub make executions or scripts it does not need to
be exported.
`#ifdef FEATURE_PERIPH_NAME` usage should now be replaced by
`#ifdef MODULE_PERIPH_NAME`.
This deprecation was started by pull requests #8226 and #8227.
