Apple build graph targets and commands

Once now supports Apple build graph targets defined in once.toml. You can declare libraries, frameworks, applications, and test bundles using built-in Apple rule metadata, inspect them with once query targets, once query capabilities, and once query schema, and run local cache-backed once build, once run, and once test commands that materialize artifacts under .once/out/<target>/. This is the first implementation of the build graph described in RFC 0001, focused on Apple platforms but not yet invoking Xcode.