Change how the dblatex/lndir problem is worked around

Change how the dblatex/lndir problem is worked around
Hack: dblatex normalises the name of the main input file using
os.path.realpath, which means that if we're in a linked build tree,
it find the real source files rather than the symlinks in our link
tree. This is fine for the static sources, but it means it can't
find the generated sources.

We therefore also generate the main input file, so that it really
is in the link tree, and thus dblatex can find everything.