added support for file:// url to repository

file:// is valid url even if it does not define a host.

allows to define a repo like this (local directory generated with
composer/satis):

```json
{
	"repositories": [ { "type": "composer", "url": "file:///home/cebe/dev/jescali/xeno-core/core/repo" } ],
	"require": {
        ...
	}
}
```
Merge pull request #2386 from cebe/allow-file-url

added support for file:// url to repository