Remove use of Rustc from cargo-test-support.
Remove use of Rustc from cargo-test-support.

cargo-test-support wasn't using any of the caching or other logic from
Rustc, so this just swaps with a very basic implementation in order to
remove the dependency on `cargo`.

rust-lang/cargo,rustyhorde/cargo,eminence/cargo,eminence/cargo,rust-lang/cargo,nipunn1313/cargo,rustyhorde/cargo,eminence/cargo,nipunn1313/cargo,nipunn1313/cargo,rustyhorde/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo,rust-lang/cargo,rust-lang/cargo