[Macros] Improve diagnostics for missing macro implementation (#65814)
We should no longer say "macro implementation must be provided via '-load-plugin-library'" as there are multiple ways to pick up a macro dependency. Here's an improved version:
> warning: external macro implementation type 'MissingModule.MissingType' could not be found for macro 'missingMacro1'; the type must be public and provided by a macro target in a Swift package, or via '-plugin-path' or '-load-plugin-library'