Avoid compilation failure with strict_variables=true

When strict_variables is on and mcollective has yet to be installed,
mco_version isn't defined, so referring to it here breaks compilation.
The fix here uses a pattern used by puppetlabs-apt.

manifests/defaults.pp

@@ -31,7 +31,10 @@
   # https://docs.puppetlabs.com/mcollective/releasenotes.html#libdirloadpath-changes-and-core-plugins
   $mco_assumed_version = '2.8.5'
 
-  $_mco_version = pick_default($::mco_version, $mco_assumed_version)
+  $_mco_version = defined('$mco_version') ? {
+    true    => $::mco_version,
+    default => $mco_assumed_version,
+  }
   if versioncmp($_mco_version, '2.8') >= 0 {
     $core_libdir = undef
   } else {