symlink: Add non-ga base product type
    
    The same layered product can be built for multiple versions of base
    product. We don't want colliding symlinks. For backwards compatibility
    the type is only added when it's not GA. This way it's also similar to
    how release IDs are generated in productmd.
    
    JIRA: RCMWORK-17923
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>