[SwiftCompilerSources] Make Package.swift static

`Package.swift` in the build directory and symlinking Sources directory
wasn't great for source control and break points. Since C modules doesn't
need the generated headers at this point, make `Package.swift` static
again with a symlink to the string processing source directory.

SwiftCompilerSources/Package.swift

@@ -0,0 +1,86 @@
+// swift-tools-version:5.3
+//===--- Package.swift.in - SwiftCompiler SwiftPM package -----------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2021 - 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import PackageDescription
+
+private extension Target {
+  static let defaultSwiftSettings: [SwiftSetting] = [
+    .unsafeFlags([
+      "-Xfrontend", "-validate-tbd-against-ir=none",
+      "-Xfrontend", "-enable-cxx-interop",
+      // Bridging modules and headers
+      "-Xcc", "-I", "-Xcc", "../include",
+      "-cross-module-optimization"
+    ]),
+  ]
+
+  static func compilerModuleTarget(
+    name: String,
+    dependencies: [Dependency],
+    path: String? = nil,
+    sources: [String]? = nil,
+    swiftSettings: [SwiftSetting] = []) -> Target {
+      .target(
+        name: name,
+        dependencies: dependencies,
+        path: path ?? "Sources/\(name)",
+        exclude: ["CMakeLists.txt"],
+        sources: sources,
+        swiftSettings: defaultSwiftSettings + swiftSettings)
+    }
+}
+
+let package = Package(
+  name: "SwiftCompilerSources",
+  platforms: [
+    .macOS("10.9"),
+  ],
+  products: [
+    .library(
+      name: "swiftCompilerModules",
+      type: .static,
+      targets: ["Basic", "AST", "Parse", "SIL", "Optimizer", "_CompilerRegexParser"]),
+  ],
+  dependencies: [
+  ],
+  // Note that targets and their dependencies must align with
+  // 'SwiftCompilerSources/Sources/CMakeLists.txt'
+  targets: [
+    .compilerModuleTarget(
+      name: "_CompilerRegexParser",
+      dependencies: [],
+      path: "_RegexParser_Sources",
+      swiftSettings: [
+        // Workaround until `_CompilerRegexParser` is imported as implementation-only
+        // by `_StringProcessing`.
+        .unsafeFlags([
+          "-Xfrontend",
+          "-disable-implicit-string-processing-module-import"
+        ])]),
+    .compilerModuleTarget(
+      name: "Basic",
+      dependencies: []),
+    .compilerModuleTarget(
+      name: "AST",
+      dependencies: ["Basic"]),
+    .compilerModuleTarget(
+      name: "Parse",
+      dependencies: ["Basic", "AST", "_CompilerRegexParser"]),
+    .compilerModuleTarget(
+      name: "SIL",
+      dependencies: ["Basic"]),
+    .compilerModuleTarget(
+      name: "Optimizer",
+      dependencies: ["Basic", "SIL", "Parse"]),
+  ]
+)