Refactor program pipeline handling. In preparation for moving more code from gl::Program to gl::ProgramExecutable so it can be shared with ProgramPipeline. Bug: angleproject:6566 Change-Id: Icb7ecccb37ae8e0d7d5fef8968f0dd7ef6fe6150 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3226305 Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>