Add feature guards for necessary requirements

Add `Puppet.feature` guards for the `yard` and `rgen` libraries.

lib/puppet/face/yardoc.rb

@@ -1,5 +1,4 @@
 require 'puppet/face'
-require 'puppet/pops'
 
 Puppet::Face.define(:yardoc, '0.0.1') do
 
@@ -7,6 +6,17 @@ Puppet::Face.define(:yardoc, '0.0.1') do
     default
 
     when_invoked do |*args|
+
+      unless Puppet.features.yard?
+        raise RuntimeError, "The 'yard' gem must be installed in order to use this face."
+      end
+
+      if Puppet.features.rgen?
+        require 'puppet/pops'
+      else
+        raise RuntimeError, "The 'rgen' gem must be installed in order to use this face."
+      end
+
       parser = Puppet::Pops::Parser::Parser.new()
       parse_result = parser.parse_file(args[0])
 

lib/puppet/feature/rgen.rb

@@ -0,0 +1,3 @@
+require 'puppet/util/feature'
+
+Puppet.features.add(:rgen, :libs => ['rgen/metamodel_builder', 'rgen/ecore/ecore'])

lib/puppet/feature/yard.rb

@@ -0,0 +1,3 @@
+require 'puppet/util/feature'
+
+Puppet.features.add(:yard, :libs => ['yard'])