* start renaming "package" to "module" (see #14307)
  - build system gains `main_mod_path` and `main_pkg_path` is still
    there but it is deprecated.
* eliminate the object-oriented memory management style of what was
  previously `*Package`. Now it is `*Package.Module` and all pointers
  point to externally managed memory.
* fixes to get the new Fetch.zig code working. The previous commit was
  work-in-progress. There are still two commented out code paths, the
  one that leads to `Compilation.create` and the one for `zig build`
  that fetches the entire dependency tree and creates the required
  modules for the build runner.