[infra] Refactors for gen_tasks

This reduces the number of things we have to remember when specifying tasks, and makes it simpler to switch between pre-built and just-in-time task drivers.

Bug: b/362481036
Change-Id: If08d1d62366b88d92fce2d11dd8903f5bfce0a66
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/894838
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
5 files changed