(PUP-987) Do not look for ruby manifests in modules

There was additional code in Puppet::Module looking up *.rb manifests in
modules.  Since Puppet 4 no longer supports Ruby DSL, this needed cleanup
as well (in addition to the early work on done on PUP-987).
(maint) Refactor match_manifest

Now that we are only looking up init.pp, we can save a few Array
creation steps and just unshift it to the front of searched if it is not
already present.