Introduce `dispatchStates` type
Introduce `dispatchStates` type

`dispatchStates` manages a collection of `dispatchState`.
By `dispatchStates`, we can aggregate how to find `dispatchState`
in one place and `dispatchStatesByName` local variable in `Dockerfile2LLB`
can be removed.

Signed-off-by: Yuichiro Kaneko <37b8411fc533112bb69dd1aa56980922c21ae5e0@gmail.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc