Failure to build 1.4.1 on existing projects that were using 1.3.2

Similar to #203, it seems like the 1.4.1 releases are still making assumptions about items in the current space that weren't prior, which is a breaking change, and fails to build on projects who were dependent on 1.3.2.

Cargo.toml:

[package]
name = "pkg"
edition = "2021"

[dependencies]
soroban-sdk = { version = "=21.7.6" }

[dev-dependencies]
soroban-sdk = { version = "=21.7.6", features = ["testutils"] }

src/lib.rs:

#![no_std]

use soroban_sdk::contracttype;

#[contracttype]
#[derive(Clone, Debug, PartialEq)]
pub struct Struct {
    pub index: u64,
}

Run cargo test with 1.4.1:

$ cargo update artbirary --precise 1.4.1
$ cargo update derive_artbirary --precise 1.4.1
$ cargo test

Included in the output are the following build errors:

error[E0433]: failed to resolve: could not find `std` in the list of imported crates
 --> src/lib.rs:5:1
  |
5 | #[contracttype]
  | ^^^^^^^^^^^^^^^ could not find `std` in the list of imported crates
  |
  = note: this error originates in the derive macro `soroban_sdk::testutils::arbitrary::arbitrary::Arbitrary` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0425]: cannot find value `RECURSIVE_COUNT_ArbitraryStruct` in this scope
 --> src/lib.rs:5:1
  |
5 | #[contracttype]
  | ^^^^^^^^^^^^^^^ not found in this scope
  |
  = note: this error originates in the derive macro `soroban_sdk::testutils::arbitrary::arbitrary::Arbitrary` (in Nightly builds, run with -Z macro-backtrace for more info)

Re-run cargo test with 1.3.2 and it compiles:

$ cargo update artbirary --precise 1.3.2
$ cargo update derive_artbirary --precise 1.3.2
$ cargo test

Reproduce the failure with the Cargo.toml and src/lib.rs examples by pinning arbitrary and derive_arbitrary to 1.4.1, then run cargo test and compare with 1.3.2. Start from the #[contracttype] expansion and the reported Arbitrary errors; done means the existing project builds successfully with 1.4.1.