* 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.
