Currently, the computation of the 'pure' name or the variant is
hardcoded and spread around the functions that need it. This is not
nice, and in the future we'd spread it even more with more usage of
variants/pure os names.

This patch abstracts these functions into the OS class, and then
replaces the hardcoded uses with the new functions.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>