Expose all BSP messages used by SourceKit-LSP to BSP servers

The interaction to an out-of-process BSP server still went through the `BuildServerBuildSystem`, which doesn’t forward all messages to the build system and uses the old push-based model for build settings.

If we discover that the BSP server supports the new pull-based build settings model, we now forward all methods to it directly, without going through `BuiltInBuildSystemAdapter`, which has been renamed to `LegacyBuildServerBuildSystem`.

rdar://136106323
rdar://127606323
rdar://126493405
Fixes #1226
Fixes #1173

Sources/BuildSystemIntegration/DetermineBuildSystem.swift

@@ -0,0 +1,58 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2024 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 LanguageServerProtocol
+import SKLogging
+import SKOptions
+import ToolchainRegistry
+
+import struct TSCBasic.AbsolutePath
+
+/// Determine which build system should be started to handle the given workspace folder and at which folder that build
+/// system's project root is (see `BuiltInBuildSystem.projectRoot(for:options:)`).
+///
+/// Returns `nil` if no build system can handle this workspace folder.
+package func determineBuildSystem(
+  forWorkspaceFolder workspaceFolder: DocumentURI,
+  options: SourceKitLSPOptions
+) -> BuildSystemKind? {
+  var buildSystemPreference: [WorkspaceType] = [
+    .buildServer, .swiftPM, .compilationDatabase,
+  ]
+  if let defaultBuildSystem = options.defaultWorkspaceType {
+    buildSystemPreference.removeAll(where: { $0 == defaultBuildSystem })
+    buildSystemPreference.insert(defaultBuildSystem, at: 0)
+  }
+  guard let workspaceFolderUrl = workspaceFolder.fileURL,
+    let workspaceFolderPath = try? AbsolutePath(validating: workspaceFolderUrl.path)
+  else {
+    return nil
+  }
+  for buildSystemType in buildSystemPreference {
+    switch buildSystemType {
+    case .buildServer:
+      if let projectRoot = ExternalBuildSystemAdapter.projectRoot(for: workspaceFolderPath, options: options) {
+        return .buildServer(projectRoot: projectRoot)
+      }
+    case .compilationDatabase:
+      if let projectRoot = CompilationDatabaseBuildSystem.projectRoot(for: workspaceFolderPath, options: options) {
+        return .compilationDatabase(projectRoot: projectRoot)
+      }
+    case .swiftPM:
+      if let projectRoot = SwiftPMBuildSystem.projectRoot(for: workspaceFolderPath, options: options) {
+        return .swiftPM(projectRoot: projectRoot)
+      }
+    }
+  }
+
+  return nil
+}