The first problem was that the list of files/dirs to embed or ignore was not
up-to-date. The second problem was that the 'Cwd' option used when running the
Tar builder in the source-dist rule didn't actually change the current directory
and was therefore failing. Finally, the source-dist rule did not pre-generate
Haskell modules derived from .x (alex) and .y (happy) files, like the Make
build system does -- this is now fixed.

We might be doing too much work for that last step (we seem to be building
many things until we get to generating the source distribution), but extracting
the distribution and running

    ./configure && hadrian/build.sh --flavour=quickest -j

from there does work for me now.