Add support for submodules in single repo source packaging

This includes submodules in the source packages if present in the repository to
package.

As this is currently only needed for enterprise packaging, it's scoped accordingly.

Change-Id: Iaf971a04a5687e2af393316b5814e0b817c49a14
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Antti Kokko <antti.kokko@digia.com>