Refactor the Blubber BuildKit frontend to use upstream's `dockerui` that
provides types/functions for:

 - Handling common gateway options (build target, platform, labels,
   etc.)
 - Parsing build arguments passed by the docker toolchain
 - Retrieval of the client build context, supporting local contexts as
   well as pulling directly from HTTP and Git sources
 - Storing provenance information from build context sources including
   information about the Git repo and HEAD commit
 - Loading of the build configuration
 - Exclusion of files from the build context based on `.dockerignore`
   files
 - A common `dockerui.Client.Build` function that creates proper
   multi-platform results given a single-platform factory function.

All code that was previously cargo cult'ed to provide some of the above
has been removed.

Refactoring of `build.Options` and `build.Target` was necessary to
facilitate retrieval of the build context via a new `ContextResolver`
interface instead of constructing LLB sources directly. The use of
dependency injection here keeps the `build` package agnostic to the
underlying use of `dockerui` by the `buildkit` package.

Change-Id: Id88813a477ebc40cac8afde347dc9d7fc5e653a0