Prevent circular dependencies in $extends

test/cli/bundle/reference/pcbs/custom_template.kicad_pcb

@@ -1 +1 @@
-Custom template override. The secret is 42. MakerJS is 0.17.0. Ergogen is 4.0.5.
+Custom template override. The secret is 42. MakerJS is loaded. Ergogen is loaded.

test/fixtures/bundle/templates/custom_template.js

@@ -4,6 +4,6 @@ const version = require('package.json').version
 module.exports = {
     convert_outline: () => {},
     body: params => {
-        return `Custom template override. The secret is ${params.custom.secret}. MakerJS is ${m.version}. Ergogen is ${version}.`
+        return `Custom template override. The secret is ${params.custom.secret}. MakerJS is ${m.version.length ? 'loaded' : ''}. Ergogen is ${version.length ? 'loaded' : ''}.`
     }
 }

test/unit/prepare.js

@@ -61,6 +61,12 @@ describe('Prepare', function() {
             y: 2,
             z: 3
         })
+        // should be able to detect circular dependencies and error out
+        p.inherit.bind(this, {
+            a: {
+                $extends: 'a'
+            }
+        }).should.throw('circular dependency')
     })
 
     it('parameterize', function() {