Rollup merge of #125913 - fmease:early-lints-spruce-up-some-diags, r=Nadrieril

Spruce up the diagnostics of some early lints

Implement the various "*(note to myself) in a follow-up PR we should turn parts of this message into a subdiagnostic (help msg or even struct sugg)*" drive-by comments I left in #124417 during my review.

For context, before #124417, only a few early lints touched/decorated/customized their diagnostic because the former API made it a bit awkward. Likely because of that, things that should've been subdiagnostics were just crammed into the primary message. This PR rectifies this.

compiler/rustc_lint_defs/src/lib.rs

@@ -706,7 +706,10 @@ pub enum BuiltinLintDiag {
     },
     MacroUseDeprecated,
     UnusedMacroUse,
-    PrivateExternCrateReexport(Ident),
+    PrivateExternCrateReexport {
+        source: Ident,
+        extern_crate_span: Span,
+    },
     UnusedLabel,
     MacroIsPrivate(Ident),
     UnusedMacroDefinition(Symbol),