[REFACTOR] Rework how changed service files are reported
[REFACTOR] Rework how changed service files are reported

First, we break out a `service_files` method, that returns an iterator
of all the group's current service files. This will be important in
coming commits, because before now, we did not have a way to access
this information.

Then, we turn `changed_service_files` into a `HashSet` and use that to
filter the list of all files.

Signed-off-by: Christopher Maier <b4598f17a90ef80817bae8ad6c37249740caafb4@chef.io>

habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat