* 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.
Most of this migration was performed automatically with `zig fmt`. There
were a few exceptions which I had to manually fix:
* `@alignCast` and `@addrSpaceCast` cannot be automatically rewritten
* `@truncate`'s fixup is incorrect for vectors
* Test cases are not formatted, and their error locations change
* improve error message when build manifest file is missing
* update std.zig.Ast to support ZON
* Compilation.AllErrors.Message: make the notes field a const slice
* move build manifest parsing logic into src/Manifest.zig and add more
checks, and make the checks integrate into the standard error
reporting code so that reported errors look sexy
closes#14290
