Release tarballs already have dist/. We can't rebuild the webpack in an
RPM as that requires npm and network access for `npm install`. But we
also don't want to ship the entire `node_modules/` as that is huge and
would require lots of careful license review.
So ship a stub node_modules/ that satisfies the Makefile dependency.
Make sure that this is not newer than package.json or anything in dist/,
so that none of the Makefile's rebuild rules trigger.
Closes#36
