Use async methods when loading manifests (#8551)

### Motivation:

This follows on from my work to convert the registry code to `async`
methods.

### Modifications:

Switch to using async methods when loading manifests, converting
synchronous code to the simpler async equivalents and converting sync
API calls to their synchronous versions.

### Result:

This resolves several warnings and FIXMEs, makes the code easier to read
and generally implements concurrency in a more idiomatic way.

Author: plemarquand

Sources/Basics/Concurrency/ConcurrencyHelpers.swift

@@ -59,4 +59,20 @@ extension DispatchQueue {
             }
         }
     }
+
+    package func asyncResult<T: Sendable>(_ callback: @escaping @Sendable (Result<T, Error>) -> Void, _ closure: @escaping @Sendable () async throws -> T) {
+        let completion: @Sendable (Result<T, Error>) -> Void = {
+            result in self.async {
+                callback(result)
+            }
+        }
+
+        Task {
+            do {
+                completion(.success(try await closure()))
+            } catch {
+                completion(.failure(error))
+            }
+        }
+    }
 }

Sources/Commands/PackageCommands/ResetCommands.swift

@@ -27,15 +27,15 @@ extension SwiftPackageCommand {
         }
     }
 
-    struct PurgeCache: SwiftCommand {
+    struct PurgeCache: AsyncSwiftCommand {
         static let configuration = CommandConfiguration(
             abstract: "Purge the global repository cache.")
 
         @OptionGroup(visibility: .hidden)
         var globalOptions: GlobalOptions
 
-        func run(_ swiftCommandState: SwiftCommandState) throws {
-            try swiftCommandState.getActiveWorkspace().purgeCache(observabilityScope: swiftCommandState.observabilityScope)
+        func run(_ swiftCommandState: SwiftCommandState) async throws {
+            try await swiftCommandState.getActiveWorkspace().purgeCache(observabilityScope: swiftCommandState.observabilityScope)
         }
     }