Add support for mirroring to sources.rb
Add support for mirroring to sources.rb

The sources.rb script computes the URL from where to download a
package's source code. It now accepts a flag such that it can take
mirroring into account.

Signed-off-by: Christian <4f88b50bc9dba63c9a7d9e208d4b3ac6f1a52800@citrix.com>

lindig/xs-opam,mseri/xs-opam,xapi-project/xs-opam,gaborigloi/xs-opam,mseri/xs-opam