l3build: allow dirs to be customised

we may not wish to keep this feature (could it cause
issues with bundles?), but I wanted to experiment.
I think I prefer having

    build/distrib
    build/local
    build/test
    build/unpacked

but I haven't changed any defaults yet.

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5022 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build: allow for missing support/ dirs, etc.

I've been experimenting with trying to minimise the
amount of additional material needed in a package directory
when using l3build. Before this commit, having 
"support/" was necessary even if empty.

This might not be the best place to make this check, though...

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5023 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build.dtx: add the lua script, verbatim, to the typeset documentation

probably don't want to use listings, long term :)

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5024 de43f980-851b-0410-b2f7-c40aca1f87e0