Using wkg to manage wit dependencies

[thomastaylor312]

Now that we have wkg (from wasm-pkg-tools) available and working for pulling wit dependencies, would people be amenable to using wkg to fetch dependencies in the wasi-* crates in this repo? The main idea here is to start using the packaging tooling in all the places where it makes sense and it would avoid needing to copy/paste interfaces around

The main changes this would entail are

• We would commit a wkg.lock file in each of the wasi crates
• Updates to contributing/developer docs to indicate that wkg should be used for updating the dependencies
• (Optional) We would no longer commit the deps directory and instead fetch at build time. This would require an update to the build pipelines. This isn't a requirement but would make things cleaner

I'm totally fine if people don't want to use this, but wanted to at least bring it up for discussion! If we do want to do this, I can do the work to enable it

[alexcrichton]

I think it'd be reasonable to do this! I think the best way to start would be to replace ci/vendor-wit.shwith wkg, so we'd still check in everything to the repo, but CI would run wkg instead of a "random bash script" to ensure that everything is vendored correctly (or the "random bash script" would be much simpler and just run wkg in a view invocations)