CRM_Extension_Browser - Allow different cache files for different feeds

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).