Fixing alias site directory lookup to check sites.php

The previous code from issue #339 did not convert the URI to a hostname before checking the sites.php file and was therefore not returning the proper result.

Instead, the function `drush_sitealias_uri_to_site_dir($uri)` should handle the lookup since its job is to return the directory for a site given its URI.

In this pull request I removed the erroneous code from the previous attempt and added the sites.php lookup to the aforementioned function.