Print reason for requiring resolution in verbose mode

This is a pretty basic translation from `WorkspaceResolveReason` into prose which may need some improvements in the future, but it's a start at least.

Author: neonichu

Sources/Workspace/Workspace.swift

@@ -2033,11 +2033,28 @@ extension Workspace {
             pinsStore: pinsStore
         )
 
-        if precomputationResult.isRequired {
+        if case let .required(result) = precomputationResult {
+            let reason: String
+            if verbosity != .concise {
+                switch result {
+                case .forced:
+                    reason = "\nResolution is required because it was forced."
+                case .newPackages(let packages):
+                    let packagesString = packages.map { $0.name }.joined(separator: ", ")
+                    reason = "\nResolution is required because there are new packages in the graph: \(packagesString)"
+                case .packageRequirementChange(let package, let state, let requirement):
+                    reason = "\nResolution is required because the requirement of package '\(package.name)' changed: \(state) => \(requirement)"
+                case .other:
+                    reason = "\nResolution is required for an unknown reason"
+                }
+            } else {
+                reason = ""
+            }
+
             if !fileSystem.exists(self.location.resolvedVersionsFile) {
-                diagnostics.emit(error: "a resolved file is required when automatic dependency resolution is disabled and should be placed at \(self.location.resolvedVersionsFile.pathString)")
+                diagnostics.emit(error: "a resolved file is required when automatic dependency resolution is disabled and should be placed at \(self.location.resolvedVersionsFile.pathString)\(reason)")
             } else {
-                diagnostics.emit(error: "an out-of-date resolved file was detected at \(self.location.resolvedVersionsFile.pathString), which is not allowed when automatic dependency resolution is disabled; please make sure to update the file to reflect the changes in dependencies")
+                diagnostics.emit(error: "an out-of-date resolved file was detected at \(self.location.resolvedVersionsFile.pathString), which is not allowed when automatic dependency resolution is disabled; please make sure to update the file to reflect the changes in dependencies\(reason)")
             }
         }