Reduce FastGettext's reads of empty.mo

While processing a request, the puppet master outputs dozens of lines like this one:

[pid  3536] open("/opt/puppetlabs/puppet/lib/ruby/gems/2.4.0/gems/fast_gettext-1.1.0/lib/fast_gettext/vendor/empty.mo", O_RDONLY|O_CLOEXEC) = 13

This is most likely coming from modules that have a locales directory but no translations for the selected locale. FastGettext will create a dummy empty translation file for it to load and read. We should see if there is a way we can avoid most of these reads.

One idea is to not create translation repositories (which load translation – or empty.mo – files lazily) for modules that don't have translations for the selected locale. We don't currently support changing the locale at runtime, so checking for the relevant locale directory before creating the repository might be sufficient.