Provide new `build` interfaces, `build.Target` and `build.Options`.

The latter, `build.Options`, moves configuration of build-time options
from the `buildkit` module into the core `build` module. New fields
are introduced that were formerly handled by `dockerfile2llb`.

The former, `build.Target`, provides a single interface to perform
operations that will either affect the image config result (entrypoint,
runtime user, etc.) or the `llb.State` graph (copy files, run commands,
etc.).

Refactor `build.Instruction` interface and implementations to accept a
single `build.Target` argument. Instead of returning half-compiled
Dockerfile instructions, each `build.Instruction` implementation uses
the passed `build.Target` to add to the `llb.State` graph or make
changes to image configuration.

Note, it may be better long-term to remove the `build.Instruction`
abstraction altogether (e.g. ostensibly we could pass the `build.Target`
directly to `config` types), but this seems like a good interim part of
the large refactoring as it avoided having to change anything in the
`config` module.

Refactor `buildkit` module to use the new `build.Target` and
`build.TargetGroup` interfaces. Instead of compiling to Dockerfile and
using `dockerfile2llb` to convert that to LLB, `build.Instruction`s are
compiled to the `build.Target` which is marshaled directly to a
`llb.Definition`.

Provide test helper modules `llbtest` and `testtarget` to ease making
complex assertions on the `llb.Definition` result of `build.Target`.

Provide test coverage for all `build.Target` methods and refactor
existing `build.Instruction` tests.

Remove `docker` module.

Bug: T345458
Change-Id: I902faaa0a5795c38aa17816e4ea157e07211e9d6