Allow overriding SwiftPM's build directory with background indexing

If `scratchPath` is explicitly set, use it over the separate
`index-build` arena even when background indexing is enabled.

Author: bnbarham

Sources/BuildSystemIntegration/SwiftPMBuildSystem.swift

@@ -203,7 +203,7 @@ package actor SwiftPMBuildSystem: BuiltInBuildSystem {
       throw Error.cannotDetermineHostToolchain
     }
 
-    var absProjectRoot = try AbsolutePath(validating: projectRoot.filePath)
+    let absProjectRoot = try AbsolutePath(validating: projectRoot.filePath)
     let hostSDK = try SwiftSDK.hostSwiftSDK(AbsolutePath(validating: destinationToolchainBinDir.filePath))
     let hostSwiftPMToolchain = try UserToolchain(swiftSDK: hostSDK)
 
@@ -237,10 +237,10 @@ package actor SwiftPMBuildSystem: BuiltInBuildSystem {
       fileSystem: localFileSystem
     )
 
-    if options.backgroundIndexingOrDefault {
-      location.scratchDirectory = absProjectRoot.appending(components: ".build", "index-build")
-    } else if let scratchDirectory = options.swiftPMOrDefault.scratchPath {
+    if let scratchDirectory = options.swiftPMOrDefault.scratchPath {
       location.scratchDirectory = try AbsolutePath(validating: scratchDirectory, relativeTo: absProjectRoot)
+    } else if options.backgroundIndexingOrDefault {
+      location.scratchDirectory = absProjectRoot.appending(components: ".build", "index-build")
     }
 
     var configuration = WorkspaceConfiguration.default

Sources/InProcessClient/InProcessSourceKitLSPClient.swift

@@ -15,7 +15,7 @@ public import Foundation
 public import LanguageServerProtocol
 import LanguageServerProtocolExtensions
 import SKLogging
-package import SKOptions
+public import SKOptions
 package import SourceKitLSP
 import SwiftExtensions
 import TSCExtensions
@@ -31,13 +31,14 @@ public final class InProcessSourceKitLSPClient: Sendable {
 
   public convenience init(
     toolchainPath: URL?,
+    options: SourceKitLSPOptions = SourceKitLSPOptions(),
     capabilities: ClientCapabilities = ClientCapabilities(),
     workspaceFolders: [WorkspaceFolder],
     messageHandler: any MessageHandler
   ) async throws {
     try await self.init(
       toolchainRegistry: ToolchainRegistry(installPath: toolchainPath),
-      options: SourceKitLSPOptions(),
+      options: options,
       capabilities: capabilities,
       workspaceFolders: workspaceFolders,
       messageHandler: messageHandler