Change .merlin generation to not use PKG
Change .merlin generation to not use PKG

We know the build directory of exteranl packages, hence we should simply use it
as the build and source dir (B and S respectively).

janestreet/jbuilder,rdavison/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,rdavison/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder