update zig, build API: module rework

[linuxy]

Addresses the updates from ziglang PR ziglang/zig#14538

[mitchellh]

Great, thank you!

[mitchellh]

I'll do a follow up unless you're interested in doing this:

I think we should do a semver check on builtin.zig_version (stdlib has all the functions) to continue exported a Pkg type so that downstream can continue to use older Zig versions. I'm interested in staying on nightly but the build system in particular is SO unstable right now that I think keeping downstream consumers pkg-aware is fine for a few weeks/months.

Selfish need because I have a downstream use of this lib and it has a bunch of other deps that aren't updated yet so I can't update myself...

[mitchellh]

Made #19 to track that.