wasmtime-wiggle-macro: adopt config system from wiggle-generate

crates/wasi/src/lib.rs

@@ -6,7 +6,9 @@ pub use wasi_common::{WasiCtx, WasiCtxBuilder};
 
 // Defines a `struct Wasi` with member fields and appropriate APIs for dealing
 // with all the various WASI exports.
-wasmtime_wiggle::define_struct_for_wiggle!("phases/snapshot/witx/wasi_snapshot_preview1.witx");
+wasmtime_wiggle::define_struct_for_wiggle!({
+    witx: ["../wasi-common/WASI/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