Make sdkRootPath property of swift-sdk.json targetTriples object optional (#8687)

Makes the "sdkRootPath" property of `swift-sdk.json` optional.

### Motivation:

The Android SDK bundle (swiftlang/swift#80788)
does not include the Android NDK's sysroot in the bundle itself, but
instead relies on it being installed locally. The install location will
vary, and the user will be able to configure it with a command (modulo
#8584) like:

```
swift sdk configure --sdk-root-path ~/Library/Android/sdk/ndk/27.0.12077973/toolchains/llvm/prebuilt/darwin-x86_64/sysroot/ swift-6.2-RELEASE-android-0.1 aarch64-unknown-linux-android28
```

However, since the `SwiftSDKMetadataV4.sdkRootPath` property is declared
as non-optional, *some* value must be included in the `swift-sdk.json`
file.

@MaxDesiatov at
swiftlang/swift#80788 (comment)
mentions:

> As for making it optional, I don't quite remember the exact issue that
caused it to become non-optional. After all, making it optional is
technically not a breaking change, so potentially could be considered if
necessary.


### Modifications:

Change `SwiftSDKMetadataV4.sdkRootPath` from `String` to `String?`

### Result:

The swift-sdk.json can now contain destinations that do not specify a
`sdkRootPath` property.

Author: marcprux

Sources/PackageModel/SwiftSDKs/SwiftSDK.swift

@@ -340,7 +340,9 @@ public struct SwiftSDK: Equatable {
             swiftSDKDirectory: Basics.AbsolutePath? = nil
         ) throws where Path == Basics.AbsolutePath {
             self.init(
-                sdkRootPath: try AbsolutePath(validating: properties.sdkRootPath, relativeTo: swiftSDKDirectory),
+                sdkRootPath: try properties.sdkRootPath.map {
+                    try AbsolutePath(validating: $0, relativeTo: swiftSDKDirectory)
+                },
                 swiftResourcesPath: try properties.swiftResourcesPath.map {
                     try AbsolutePath(validating: $0, relativeTo: swiftSDKDirectory)
                 },
@@ -1175,7 +1177,7 @@ struct SerializedDestinationV3: Decodable {
 struct SwiftSDKMetadataV4: Decodable {
     struct TripleProperties: Codable {
         /// Path relative to `swift-sdk.json` containing SDK root.
-        var sdkRootPath: String
+        var sdkRootPath: String?
 
         /// Path relative to `swift-sdk.json` containing Swift resources for dynamic linking.
         var swiftResourcesPath: String?

Tests/PackageModelTests/SwiftSDKTests.swift

@@ -25,6 +25,7 @@ private let hostTriple = try! Triple("arm64-apple-darwin22.1.0")
 private let olderHostTriple = try! Triple("arm64-apple-darwin20.1.0")
 private let linuxGNUTargetTriple = try! Triple("x86_64-unknown-linux-gnu")
 private let linuxMuslTargetTriple = try! Triple("x86_64-unknown-linux-musl")
+private let androidTargetTriple = try! Triple("aarch64-unknown-linux-android28")
 private let wasiTargetTriple = try! Triple("wasm32-unknown-wasi")
 private let extraFlags = BuildFlags(
     cCompilerFlags: ["-fintegrated-as"],
@@ -186,6 +187,20 @@ private let toolsetRootSwiftSDKv4 = (
     """# as SerializedJSON
 )
 
+private let androidWithoutSDKRootPathSwiftSDKv4 = (
+    path: bundleRootPath.appending(component: "androidWithoutSDKRootPathSwiftSDKv4.json"),
+    json: #"""
+    {
+        "targetTriples": {
+            "\#(androidTargetTriple.tripleString)": {
+                "toolsetPaths": ["/tools/otherToolsNoRoot.json"]
+            }
+        },
+        "schemaVersion": "4.0"
+    }
+    """# as SerializedJSON
+)
+
 private let missingToolsetSwiftSDKv4 = (
     path: bundleRootPath.appending(component: "missingToolsetSwiftSDKv4.json"),
     json: #"""
@@ -351,6 +366,23 @@ private let parsedToolsetRootDestination = SwiftSDK(
     )
 )
 
+private let parsedToolsetNoSDKRootPathDestination = SwiftSDK(
+    targetTriple: androidTargetTriple,
+    toolset: .init(
+        knownTools: [
+            .librarian: .init(path: try! AbsolutePath(validating: "\(usrBinTools[.librarian]!)")),
+            .linker: .init(path: try! AbsolutePath(validating: "\(usrBinTools[.linker]!)")),
+            .debugger: .init(path: try! AbsolutePath(validating: "\(usrBinTools[.debugger]!)")),
+        ],
+        rootPaths: []
+    ),
+    pathsConfiguration: .init(
+        sdkRootPath: nil,
+        toolsetPaths: ["/tools/otherToolsNoRoot.json"]
+            .map { try! AbsolutePath(validating: $0) }
+    )
+)
+
 private let testFiles: [(path: AbsolutePath, json: SerializedJSON)] = [
     destinationV1,
     destinationV2,
@@ -365,6 +397,7 @@ private let testFiles: [(path: AbsolutePath, json: SerializedJSON)] = [
     invalidVersionSwiftSDKv4,
     invalidToolsetSwiftSDKv4,
     wasiWithoutToolsetsSwiftSDKv4,
+    androidWithoutSDKRootPathSwiftSDKv4,
     otherToolsNoRoot,
     someToolsWithRoot,
     invalidToolset,
@@ -488,6 +521,15 @@ final class SwiftSDKTests: XCTestCase {
 
         XCTAssertEqual(toolsetRootSwiftSDKv4Decoded, [parsedToolsetRootDestination])
 
+        let androidWithoutSDKRootPathSwiftSDKv4Decoded = try SwiftSDK.decode(
+            fromFile: androidWithoutSDKRootPathSwiftSDKv4.path,
+            hostToolchainBinDir: toolchainBinAbsolutePath,
+            fileSystem: fs,
+            observabilityScope: observability
+        )
+
+        XCTAssertEqual(androidWithoutSDKRootPathSwiftSDKv4Decoded, [parsedToolsetNoSDKRootPathDestination])
+
         XCTAssertThrowsError(try SwiftSDK.decode(
             fromFile: missingToolsetSwiftSDKv4.path,
             hostToolchainBinDir: toolchainBinAbsolutePath,