Move back to only one WASI submodule (#1434)

* Move back to only one WASI submodule

This commit fixes the issue where we have two WASI submodules for build
reasons in this repository. The fix was to place the submodule in the
`wasi-common` crate, and then anyone using the `wig` crate has to be
sure to define a `WASI_ROOT` env var in a build script to be able to
parse witx files.

With all that in place `wasi-common` becomes the source of truth for the
witx files we're parsing, and crates like `wasmtime-wasi` use
build-scripts shenanigans to read the same witx files. This should
hopefully get us so we're compatible with publishing and still only have
one submodule!

* rustfmt

crates/wasi/src/lib.rs

@@ -4,9 +4,7 @@ pub use wasi_common::{WasiCtx, WasiCtxBuilder};
 
 // Defines a `struct Wasi` with member fields and appropriate APIs for dealing
 // with all the various WASI exports.
-wig::define_wasi_struct_for_wiggle!(
-    "snapshot" "wasi_snapshot_preview1"
-);
+wig::define_wasi_struct_for_wiggle!("phases/snapshot/witx/wasi_snapshot_preview1.witx");
 
 pub fn is_wasi_module(name: &str) -> bool {
     // FIXME: this should be more conservative, but while WASI is in flux and