PackageLoading: convert ManifestLoader to async/await

This work is incomplete as it causes some of the tests to hang on macOS.

Author: MaxDesiatov

Sources/Basics/ImportScanning.swift

@@ -73,4 +73,14 @@ public struct SwiftcImportScanner: ImportScanner {
                 }
             }
     }
+
+    public func scanImports(_ filePathToScan: AbsolutePath) async throws -> [String] {
+        let cmd = [swiftCompilerPath.pathString,
+                   filePathToScan.pathString,
+                   "-scan-dependencies", "-Xfrontend", "-import-prescan"] + self.swiftCompilerFlags
+
+        let result = try await TSCBasic.Process.popen(arguments: cmd, environment: self.swiftCompilerEnvironment)
+        let stdout = try result.utf8Output()
+        return try JSONDecoder.makeWithDefaults().decode(Imports.self, from: stdout).imports
+    }
 }