Ignore _self_ in the Defaults List of in Hydra 1.0

[omry]

Hydra 1.1 introduces the _self_ keyword (see Composition order).
It also changes the default composition order and suggests that users add _self_ as the first item in their defaults list to retain the old composition order.

This presents a problem for users that want their config to (temporarily) be compatible with both Hydra 1.0 and 1.1 Hydra 1.0 does not recognize _self_ and issues an error.

To make migration easier, I will make Hydra 1.0 (as of 1.0.7) ignore the _self_ keyword. Note that this will only produce the same result as Hydra 1.1 if _self_ is the first item in the Defaults List.