Move common code to gp_common and use pkg_dir

Signed-off-by: David Greaves <david.greaves@jollamobile.com>
Use a sensible tarball name for git types

Signed-off-by: David Greaves <david.greaves@jollamobile.com>
The command : git pkg    will now ensure the pkg_dir is setup if appropriate
Also support pkg_dir != rpm/

Signed-off-by: David Greaves <david.greaves@jollamobile.com>
Clean up handling of metadata from packaging file

Signed-off-by: David Greaves <david.greaves@jollamobile.com>