CRM_Extension_Browser - Allow different cache files for different feeds

The motivation here is to support these two different CLI commands:

```
cv dl myextension
cv dl myextension --dev
```

The two commands differ in that they pull from different feeds -- e.g.  the
stable feed `https://civicrm.org/extdir/ver={ver}|cms={uf}` vs the
developmental feed `https://civicrm.org/extdir/ver={ver}|cms={uf}|status=`.
However, this creates the real possibility that the user might go back/forth
among different feeds (omitting/enabling the `--dev` option per whim).

CRM/Extension/Browser.php

@@ -216,7 +216,7 @@ private function _discoverRemote() {
    * @return string
    */
   private function grabCachedJson() {
-    $filename = $this->cacheDir . DIRECTORY_SEPARATOR . self::CACHE_JSON_FILE;
+    $filename = $this->cacheDir . DIRECTORY_SEPARATOR . self::CACHE_JSON_FILE . '.' . md5($this->getRepositoryUrl());
     $json = file_get_contents($filename);
     if (empty($json)) {
       $json = $this->grabRemoteJson();
@@ -246,7 +246,7 @@ private function grabRemoteJson() {
       return array();
     }
 
-    $filename = $this->cacheDir . DIRECTORY_SEPARATOR . self::CACHE_JSON_FILE;
+    $filename = $this->cacheDir . DIRECTORY_SEPARATOR . self::CACHE_JSON_FILE . '.' . md5($this->getRepositoryUrl());
     $url = $this->getRepositoryUrl() . $this->indexPath;
     $status = CRM_Utils_HttpClient::singleton()->fetch($url, $filename);