Load each sub-module of expl3 using a separate \RequirePackage line

This will make life easier for the upcoming 'generic' loading option: by
avoiding a comma-separated list here, minimal emulation does not need to deal
with loops.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4696 de43f980-851b-0410-b2f7-c40aca1f87e0