Sort of revert 1d2b56c6 but it actually
finish the work done in ticket #9367 by removing the last recursive
makefile target in modules/.

It allows faster make (though not significant here) but most of all,
sharing the same variable definition scope in modules/ for all
makefiles.

In particular, this facilitate for future work implementing partial
linking at the module level, which actually needs the list of all
plugins being compiled.