no need to use variables in the file paths.

It buys us nothing, and causes interpolation when not strictly necessary

manifests/augeas.pp

+#class postfix::augeas
+#  This class provides the augeas lenses used by the postfix class
+#
 class postfix::augeas {
   augeas::lens {'postfix_transport':
     ensure      => present,
-    lens_source => "puppet:///modules/${module_name}/lenses/postfix_transport.aug",
-    test_source => "puppet:///modules/${module_name}/lenses/test_postfix_transport.aug",
+    lens_source => 'puppet:///modules/postfix/lenses/postfix_transport.aug',
+    test_source => 'puppet:///modules/postfix/lenses/test_postfix_transport.aug',
     stock_since => '1.0.0',
   }
   augeas::lens {'postfix_virtual':
     ensure      => present,
-    lens_source => "puppet:///modules/${module_name}/lenses/postfix_virtual.aug",
-    test_source => "puppet:///modules/${module_name}/lenses/test_postfix_virtual.aug",
+    lens_source => 'puppet:///modules/postfix/lenses/postfix_virtual.aug',
+    test_source => 'puppet:///modules/postfix/lenses/test_postfix_virtual.aug',
     stock_since => '1.0.0',
   }
 }