Don't let fuzz targets import arbitrary directly (#4806)

The version of the `arbitrary` crate used in fuzz targets needs to be
the same as the version used in `libfuzzer-sys`. That's why the latter
crate re-exports the former.

But we need to make sure to consistently use the re-exported version.
That's most easily done if that's the only version we have available.
However, `fuzz/Cargo.toml` declared a direct dependency on `arbitrary`,
making it available for import, and leading to that version being used
in a couple places.

There were two copies of `arbitrary` built before, even though they were
the same version: one with the `derive` feature turned on, through the
direct dependency, and one with it turned off when imported through
`libfuzzer-sys`. So I haven't specifically tested this but fuzzer builds
might be slightly faster now.

I have not removed the build-dep on `arbitrary`, because `build.rs` is
not invoked by libFuzzer and so it doesn't matter what version of
`arbitrary` it uses.

Our other crates, like `cranelift-fuzzgen` and `wasmtime-fuzzing`, can
still accidentally use a different version of `arbitrary` than the fuzz
targets which rely on them. This commit only fixes the direct cases
within `fuzz/**`.

fuzz/build.rs

@@ -121,15 +121,15 @@ mod component {
 
         let module = quote! {
             #[allow(unused_imports)]
-            fn static_component_api_target(input: &mut arbitrary::Unstructured) -> arbitrary::Result<()> {
+            fn static_component_api_target(input: &mut libfuzzer_sys::arbitrary::Unstructured) -> libfuzzer_sys::arbitrary::Result<()> {
                 use anyhow::Result;
-                use arbitrary::{Unstructured, Arbitrary};
-                use component_test_util::{self, Float32, Float64};
                 use component_fuzz_util::Declarations;
+                use component_test_util::{self, Float32, Float64};
+                use libfuzzer_sys::arbitrary::{self, Arbitrary};
+                use std::borrow::Cow;
                 use std::sync::{Arc, Once};
                 use wasmtime::component::{ComponentType, Lift, Lower};
                 use wasmtime_fuzzing::generators::component_types;
-                use std::borrow::Cow;
 
                 const SEED: u64 = #seed;