Enable Swift 6 mode when available, sync QueryEngine sources (#136)

As SwiftPM is more actively developed, let's consider it to be the source of truth and vendor `QueryEngine` back into `swift-sdk-generator` together with more fresh `AsyncFileSystem` code, storing all those files in `Source/Helpers/Vendor` subdirectory.

All this allows us to fix concurrency errors that prevent it from building in Swift 6 mode.

Author: MaxDesiatov

Package.swift

@@ -34,7 +34,6 @@ let package = Package(
         .product(name: "NIOHTTP1", package: "swift-nio"),
         .product(name: "Logging", package: "swift-log"),
         .product(name: "SystemPackage", package: "swift-system"),
-        "GeneratorEngine",
         "Helpers",
       ],
       exclude: ["Dockerfiles"],
@@ -51,24 +50,20 @@ let package = Package(
         .enableExperimentalFeature("StrictConcurrency=complete"),
       ]
     ),
-    .target(
-      name: "GeneratorEngine",
-      dependencies: [
-        .product(name: "Crypto", package: "swift-crypto"),
-        .product(name: "Logging", package: "swift-log"),
-        .product(name: "SystemPackage", package: "swift-system"),
-        "Helpers",
-        "SystemSQLite",
-      ]
-    ),
     .testTarget(
       name: "GeneratorEngineTests",
       dependencies: [
-        "GeneratorEngine",
+        "Helpers",
       ]
     ),
     .target(
       name: "Helpers",
+      dependencies: [
+        "SystemSQLite",
+        .product(name: "Crypto", package: "swift-crypto"),
+        .product(name: "Logging", package: "swift-log"),
+        .product(name: "SystemPackage", package: "swift-system"),
+      ],
       exclude: ["Vendor/README.md"],
       swiftSettings: [
         .enableExperimentalFeature("StrictConcurrency=complete"),
@@ -103,7 +98,8 @@ let package = Package(
         .product(name: "Logging", package: "swift-log"),
       ]
     ),
-  ]
+  ],
+  swiftLanguageVersions: [.v5, .version("6")]
 )
 
 struct Configuration {
@@ -124,7 +120,7 @@ if configuration.useAsyncHttpClient {
   package.dependencies.append(
     .package(url: "https://github.com/swift-server/async-http-client.git", from: "1.19.0")
   )
-  let targetsToAppend: Set<String> = ["SwiftSDKGenerator"]
+  let targetsToAppend: Set<String> = ["SwiftSDKGenerator", "Helpers"]
   for target in package.targets.filter({ targetsToAppend.contains($0.name) }) {
     target.dependencies.append(
       .product(name: "AsyncHTTPClient", package: "async-http-client")

Sources/AsyncProcess/ProcessExecutor.swift

@@ -119,10 +119,10 @@ private struct OutputConsumptionState: OptionSet {
 }
 
 /// Type-erasing type analogous to `AnySequence` from the Swift standard library.
-private struct AnyAsyncSequence<Element>: AsyncSequence {
-  private let iteratorFactory: () -> AsyncIterator
+private struct AnyAsyncSequence<Element>: AsyncSequence & Sendable where Element: Sendable {
+  private let iteratorFactory: @Sendable () -> AsyncIterator
 
-  init<S: AsyncSequence>(_ asyncSequence: S) where S.Element == Element {
+  init<S: AsyncSequence & Sendable>(_ asyncSequence: S) where S.Element == Element {
     self.iteratorFactory = {
       var iterator = asyncSequence.makeAsyncIterator()
       return AsyncIterator { try await iterator.next() }