source-bundle: generated script fails when a build element has no source

If a build element doesn't have any sources, then the tarball won't contain a source directory for that element but the generate build- script will attempt to copy the contents of the directory to /buildstream/build and will error out due to set -e.

The simplest fix would seem to be to check for the existence of the source directory before attempting the copy in the build-module.sh.in template, but the exact shape of that change depends on the choice of fix to issue #67 (closed).

(Perhaps I should be using kind script instead of kind manual for an element without sources, but I think that these elements are omitted from source-bundle generated tarballs?)