Add default config source to Config class

src/Composer/Config.php

@@ -84,6 +84,7 @@ class Config
     private $configSource;
     /** @var ConfigSourceInterface */
     private $authConfigSource;
+    private $defaultsConfigSource;
     private $useEnvironment;
     private $warnedHosts = array();
 
@@ -120,6 +121,16 @@ class Config
         return $this->authConfigSource;
     }
 
+    public function setDefaultsConfigSource(ConfigSourceInterface $source)
+    {
+        $this->defaultsConfigSource = $source;
+    }
+
+    public function getDefaultsConfigSource()
+    {
+        return $this->defaultsConfigSource;
+    }
+
     /**
      * Merges new config values with the existing ones (overriding)
      *