replace explicit symlinks with an autogenerated ones

previously we've had symlinks in spec/fixtures. Some operating systems
don't deal well with those, and as a consequence, our rspec suite has
been changed to verify whether a module contains symlinks. Even though
this module does not currently work on any platform that doesn't fully
support symlinks, we should be a good citizen and remove them.
.fixtures.yml has a mechanism to create them automatically, so we use
that now.

.fixtures.yml

@@ -5,3 +5,6 @@ fixtures:
     stdlib: 'https://github.com/puppetlabs/puppetlabs-stdlib'
     boolean: 'https://github.com/adrienthebo/puppet-boolean'
     filemapper: 'https://github.com/adrienthebo/puppet-filemapper'
+  symlinks:
+    network: "#{source_dir}"
+