Unzip: Check for an index before accessing it.

If there are no subdirectories, indexing into the `subdirs` array
will blow up. Let's check that there is exactly one subdirectory
of the unzipped repo before trying to access that subdirectory.
Download submodules.

After installing core quickstart, we still need to download
all the Puppet (sub)modules. This downloads them and installs
them in the proper directory.