This arranges the hak environment target info around target IDs that come from
the builder, which simplifies cross-compiling. The `target.js` module is a
generated copy of the builder's `target.ts`.
The hak environment's `target` is more like a runtime version, so this renames
to call it that, especially since we're about to add more traditional target
strings. `npm_config_target` remains, since that's what `npm` expects.
