Prevent manifest load errors from causing SwiftPM to display "fatalError" in CLI (#3261)

Looks like `Diagnostics.fatalError` is used to throw errors when appropriate diagnostics have already been emitted and the only goal is to quickly return to the caller.  This is documented in TSCUtility.

Future cleanup should avoid this special behavior.

rdar://74263826

Author: abertelrud

Sources/Workspace/Workspace.swift

@@ -1486,7 +1486,13 @@ extension Workspace {
 
                     switch result {
                     case .failure(let error):
-                        diagnostics.emit(error)
+                        // Diagnostics.fatalError indicates that a more specific diagnostic has already been added.
+                        switch error {
+                        case Diagnostics.fatalError:
+                            break
+                        default:
+                            diagnostics.emit(error)
+                        }
                     case .success(let manifest):
                         self.delegate?.didLoadManifest(packagePath: packagePath, url: url, version: version, packageKind: packageKind, manifest: manifest, diagnostics: manifestDiagnostics.diagnostics)
                     }