Engine accepts paths to custom ruleset

When PHPMD gets run, it accepts rule names, e.g.
codesize,unusedcode,naming

as a comma separated string.

It also accepts paths to a custome rule defined by the user mixed
in. For example:

```
  phpmd:
    enabled: true
    config:
      rulesets: "unusedcode,codesize,cleancode,design,naming,phpmd.xml"
```
where `phpmd.xml` contains a custom rule set up by user.

Previously, our engine failed to honor that config setup because it didn't
prefix `/code/` to the path. The result was 0 issues found across board.

This change fixes that behavior.

Author: ABaldwinHunter

Runner.php

@@ -9,6 +9,8 @@
 
 class Runner
 {
+    const RULESETS = 'cleancode,codesize,controversial,design,naming,unusedcode';
+
     private $config;
     private $server;
 
@@ -63,6 +65,19 @@ public function queuePaths($dir, $prefix = '', $exclusions = []) {
         }
     }
 
+    public function prefixCodeDirectory($configRulesets) {
+        $officialPhpRulesets = explode(',', Runner::RULESETS);
+        $configRulesets = explode(',', $configRulesets);
+
+        foreach ($configRulesets as &$r) {
+            if (!in_array($r, $officialPhpRulesets)) {
+             $r  = "/code/$r";
+            }
+        }
+
+        return implode(',', $configRulesets);
+    }
+
     public function run($files)
     {
         $resultFile = tempnam(sys_get_temp_dir(), 'phpmd');
@@ -78,10 +93,11 @@ public function run($files)
             $phpmd->setFileExtensions(explode(',', $this->config['config']['file_extensions']));
         }
 
-        $rulesets = "cleancode,codesize,controversial,design,naming,unusedcode";
+        $rulesets = Runner::RULESETS;
 
         if (isset($this->config['config']['rulesets'])) {
             $rulesets = $this->config['config']['rulesets'];
+            $rulesets = $this->prefixCodeDirectory($rulesets);
         }
 
         $phpmd->processFiles(