wit: Comments in deps don't round-trip through binary encoding

[lann]

$ find . -type f
./root.wit
./deps/dep.wit

$ wasm-tools component wit .
package pkg:root;

/// Root comment
world w {
  import pkg:dep/iface;
}
package pkg:dep {
  interface iface {
    /// Dep comment
    f: func();
  }
}

$ wasm-tools component wit . --wasm | wasm-tools component wit
package pkg:root;

/// Root comment
world w {
  import pkg:dep/iface;
}
package pkg:dep {
  interface iface {
    f: func();
  }
}

Note /// Dep comment present in the WIT is missing when round-tripped with --wasm.

[alexcrichton]

This came up as well in the discussion of #2532 -- this is a limitation of the current format of doc comments where it's per-package and there's nowhere currently to put documentation for different packages. I wrote up some more here but it'll be a bit difficult to transition the current format to preserving this.