List subclasses for each host class

Basically a straight port of the logic from the YARD template for Ruby classes.

lib/puppetx/yardoc/yard/templates/default/hostclass/html/setup.rb

@@ -1,5 +1 @@
 include T('default/module/html')
-
-def init
-  sections :header, :box_info, :pre_docstring, T('docstring'), :children
-end

lib/puppetx/yardoc/yard/templates/default/hostclass/html/subclasses.erb

@@ -0,0 +1,4 @@
+<div id="subclasses">
+  <h2>Direct Known Subclasses</h2>
+  <p class="children"><%= @subclasses.map {|child| linkify(child, child.path) }.join(", ") %></p>
+</div>

lib/puppetx/yardoc/yard/templates/default/hostclass/setup.rb

@@ -0,0 +1,19 @@
+include T('default/module')
+
+def init
+  sections :header, :box_info, :pre_docstring, T('docstring'), :subclasses
+end
+
+def subclasses
+  # The naming is a bit weird because Ruby classes use `globals.subclasses`.
+  unless globals.hostsubclasses
+    globals.hostsubclasses = {}
+    list = run_verifier Registry.all(:hostclass)
+    list.each {|o| (globals.hostsubclasses[o.parent_class.path] ||= []) << o if o.parent_class }
+  end
+
+  @subclasses = globals.hostsubclasses[object.path]
+
+  return if @subclasses.nil? || @subclasses.empty?
+  erb(:subclasses)
+end