resolve: Scale back unloading of speculatively loaded crates Fixes https://github.com/rust-lang/rust/issues/120830 and fixes https://github.com/rust-lang/rust/issues/120909 while still unblocking https://github.com/rust-lang/rust/pull/117772. I cannot reproduce https://github.com/parasyte/crash-rustc as an UI test for some reason, but I tested all the cases linked above manually.