Move code generators to codegen/ directory.

We are up to having seven distinct types of codegen, and will soon have
an 8th (DSL C++).

Change-Id: I6758328390c234ba1d5c30c118199dbc820af52a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/395817
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
31 files changed