Create mountpoint for locale files on master

The first step to being able to pluginsync module locale files from the master to the agent is to add a new mount in file_serving/mount to find locale files in modules on the master. This mount point should be located at puppet:///locales and serve up the locales/ directory of each module, when present (not all modules have them). This is analogous to the existing plugins and pluginfacts mounts. You will likely have to add some methods to the module class to do things like detecting if there is a locale directory, and returning the path to it if it exists (we are currently only referencing it here.

Some documentation about server mountpoints and puppet:/// URIs can be found here: https://puppet.com/docs/puppet/5.3/file_serving.html

Mount point tests live here https://github.com/puppetlabs/puppet/tree/master/spec/unit/file_serving/mount and should be augmented to exercise this new mountpoint.