apple_library targets now compile through the Starlark rule implementation

Once now builds apple_library targets through a real Starlark rule implementation instead of a placeholder shell script. When you run once build against an apple_library target, the tool walks dependencies in order, composes a swiftc command line via the built-in prelude, and emits a static archive and swiftmodule directory. Builds are content-addressed, so changing source files, switching Xcode versions, or editing a transitive dependency invalidates the cache entry and rebuilds only what is necessary. The new rule also resolves imports across apple_library dependencies by forwarding each dependency’s swiftmodule directory through the search path.