(maint) Use forge_modules in .fixtures.yml

Replaces use of the Git repos for stdlib and concat in .fixtures.yml to
instead pull from the Forge. This is faster, results in a lot less data
transferred, and is arguably better for testing.

.fixtures.yml

@@ -1,6 +1,6 @@
 fixtures:
-  repositories:
-    concat: 'https://github.com/puppetlabs/puppetlabs-concat'
-    stdlib: 'https://github.com/puppetlabs/puppetlabs-stdlib'
+  forge_modules:
+    concat: puppetlabs-concat
+    stdlib: puppetlabs-stdlib
   symlinks:
    "squid": "#{source_dir}"