Use SwiftPM's SDK computation logic (#1643)

Using `SwiftSDK.deriveTargetSwiftSDK`, which is the same method that SwiftPM's own CLI tools use to determine the SDK from passed-in info (target `--triple`, `--swift-sdk`, and host sdk). This allows us to better uphold the contract in the [Configuration File](d11c101ce2/Documentation/Configuration%20File.md (structure)) docs, namely that the `swiftSDK` param is "Equivalent to SwiftPM's `--swift-sdk` option" and similarly for `triple`.

As concrete examples of where (AFAICT) the current implementation diverges:
- Passing a `--triple` of `wasm32-unknown-wasi` to `swift-build` will use the toolchain-integrated Wasm SDK if one exists. Passing the same value to sourcekit-lsp does not do this.
- Perhaps more relevant: after landing https://github.com/swiftlang/swift-package-manager/pull/6828, this change will make it so that building for iOS is as simple as setting `"triple": "arm64-apple-ios"` in the config! Currently, it's necessary to set C/Swift flags and hardcode the sysroot. Should close https://github.com/swiftlang/sourcekit-lsp/issues/1587.

This PR depends on:

- https://github.com/swiftlang/swift-package-manager/pull/7925

Sources/BuildSystemIntegration/SwiftPMBuildSystem.swift

@@ -226,25 +226,23 @@ package actor SwiftPMBuildSystem {
     let hostSDK = try SwiftSDK.hostSwiftSDK(AbsolutePath(destinationToolchainBinDir))
     let hostSwiftPMToolchain = try UserToolchain(swiftSDK: hostSDK)
 
-    var destinationSDK: SwiftSDK
-    if let swiftSDK = options.swiftPM.swiftSDK {
-      let bundleStore = try SwiftSDKBundleStore(
+    let destinationSDK = try SwiftSDK.deriveTargetSwiftSDK(
+      hostSwiftSDK: hostSDK,
+      hostTriple: hostSwiftPMToolchain.targetTriple,
+      customCompileTriple: options.swiftPM.triple.map { try Triple($0) },
+      swiftSDKSelector: options.swiftPM.swiftSDK,
+      store: SwiftSDKBundleStore(
         swiftSDKsDirectory: fileSystem.getSharedSwiftSDKsDirectory(
           explicitDirectory: options.swiftPM.swiftSDKsDirectory.map { try AbsolutePath(validating: $0) }
         ),
         fileSystem: fileSystem,
         observabilityScope: observabilitySystem.topScope,
         outputHandler: { _ in }
-      )
-      destinationSDK = try bundleStore.selectBundle(matching: swiftSDK, hostTriple: hostSwiftPMToolchain.targetTriple)
-    } else {
-      destinationSDK = hostSDK
-    }
+      ),
+      observabilityScope: observabilitySystem.topScope,
+      fileSystem: fileSystem
+    )
 
-    if let triple = options.swiftPM.triple {
-      destinationSDK = hostSDK
-      destinationSDK.targetTriple = try Triple(triple)
-    }
     let destinationSwiftPMToolchain = try UserToolchain(swiftSDK: destinationSDK)
 
     var location = try Workspace.Location(

Tests/BuildSystemIntegrationTests/SwiftPMBuildSystemTests.swift

@@ -288,6 +288,46 @@ final class SwiftPMBuildSystemTests: XCTestCase {
     }
   }
 
+  func testDefaultSDKs() async throws {
+    let fs = localFileSystem
+    try await withTestScratchDir { tempDir in
+      try fs.createFiles(
+        root: tempDir,
+        files: [
+          "pkg/Sources/lib/a.swift": "",
+          "pkg/Package.swift": """
+          // swift-tools-version:6.0
+          import PackageDescription
+          let package = Package(
+            name: "a",
+            targets: [.target(name: "lib")]
+          )
+          """,
+        ]
+      )
+      let tr = ToolchainRegistry.forTesting
+
+      let options = SourceKitLSPOptions.SwiftPMOptions(
+        swiftSDKsDirectory: "/tmp/non_existent_sdks_dir",
+        triple: "wasm32-unknown-wasi"
+      )
+
+      let swiftpmBuildSystem = try await SwiftPMBuildSystem(
+        workspacePath: tempDir.appending(component: "pkg"),
+        toolchainRegistry: tr,
+        fileSystem: fs,
+        options: SourceKitLSPOptions(swiftPM: options),
+        testHooks: SwiftPMTestHooks()
+      )
+      let path = await swiftpmBuildSystem.destinationBuildParameters.toolchain.sdkRootPath
+      XCTAssertEqual(
+        path?.components.suffix(3),
+        ["usr", "share", "wasi-sysroot"],
+        "SwiftPMBuildSystem should share default SDK derivation logic with libSwiftPM"
+      )
+    }
+  }
+
   func testManifestArgs() async throws {
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in