Rollup merge of #74071 - petrochenkov:cload3, r=matthewjasper

rustc_metadata: Make crate loading fully speculative

Instead of reporting `span_err`s on the spot crate loading errors are now wrapped into the `CrateError` enum and returned, so they are reported only at the top level `resolve_crate` call, and not reported at all if we are resolving speculatively with `maybe_resolve_crate`.

As a result we can attempt loading crates for error recovery (e.g. import suggestions) without any risk of producing extra errors.
Also, this means better separation between error reporting and actual logic.

Fixes #55103
Fixes #56590

Author: Manishearth

src/librustc_error_codes/error_codes.rs

@@ -554,7 +554,7 @@ E0770: include_str!("./error_codes/E0770.md"),
 //  E0420, merged into 532
 //  E0421, merged into 531
 //  E0427, merged into 530
-    E0456, // plugin `..` is not available for triple `..`
+//  E0456, // plugin `..` is not available for triple `..`
     E0457, // plugin `..` only found in rlib format, but must be available...
     E0460, // found possibly newer version of crate `..`
     E0461, // couldn't find crate `..` with expected target triple ..