Add support for top-level variant IDs with dashes.
This is for layered products that have a variant mapped
to multiple variants in a base product, for example:
* Foo-Tools (id: FooTools, uid: Foo-Tools, name: Tools)
* Bar-Tools (id: BarTools, uid: Bar-Tools, name: Tools)
Requires productmd >= 1.2.
Signed-off-by: Daniel Mach <dmach@redhat.com>