[PackageModel] Drop a trailing \ from SDKROOT environment variable

This is a problem on windows because `platform` path is deduced
based on `parentDirectory` and `PathCchRemoveFileSpec` function
used on Windows handles it as if it was a directory.

Author: xedin

Sources/PackageModel/UserToolchain.swift

@@ -462,7 +462,7 @@ public final class UserToolchain: Toolchain {
                 // Windows uses a variable named SDKROOT to determine the root of
                 // the SDK.  This is not the same value as the SDKROOT parameter
                 // in Xcode, however, the value represents a similar concept.
-                if let SDKROOT = environment["SDKROOT"], let sdkroot = try? AbsolutePath(validating: SDKROOT) {
+                if let sdkroot = getWindowsSDKRootDirectory(from: environment) {
                     var runtime: [String] = []
                     var xctest: [String] = []
                     var swiftTesting: [String] = []
@@ -700,7 +700,7 @@ public final class UserToolchain: Toolchain {
         }
 
         if triple.isWindows() {
-            if let SDKROOT = environment["SDKROOT"], let root = try? AbsolutePath(validating: SDKROOT) {
+            if let root = Self.getWindowsSDKRootDirectory(from: environment) {
                 if let settings = WindowsSDKSettings(
                     reading: root.appending("SDKSettings.plist"),
                     observabilityScope: nil,
@@ -873,19 +873,28 @@ public final class UserToolchain: Toolchain {
         return .none
     }
 
+    private static func getWindowsSDKRootDirectory(from environment: Environment) -> AbsolutePath? {
+        if let SDKROOT = environment["SDKROOT"], let sdkDir = try? AbsolutePath(validating: SDKROOT.removingTrailingBackslash) {
+            return sdkDir
+        }
+        return nil
+    }
+
     private static func getWindowsPlatformInfo(
         swiftSDK: SwiftSDK,
         environment: Environment,
         fileSystem: any FileSystem
     ) -> (AbsolutePath, WindowsPlatformInfo)? {
-        let sdkRoot: AbsolutePath
-
-        if let sdkDir = swiftSDK.pathsConfiguration.sdkRootPath {
-            sdkRoot = sdkDir
-        } else if let SDKROOT = environment["SDKROOT"], let sdkDir = try? AbsolutePath(validating: SDKROOT) {
-            sdkRoot = sdkDir
+        let sdkRoot: AbsolutePath? = if let sdkDir = swiftSDK.pathsConfiguration.sdkRootPath {
+            sdkDir
+        } else if let sdkDir = getWindowsSDKRootDirectory(from: environment) {
+            sdkDir
         } else {
-            return .none
+            nil
+        }
+
+        guard let sdkRoot else {
+            return nil
         }
 
         // The layout of the SDK is as follows:
@@ -1079,3 +1088,13 @@ public final class UserToolchain: Toolchain {
         return `default`
     }
 }
+
+extension String {
+    fileprivate var removingTrailingBackslash: String {
+        if self.hasSuffix("\\")  {
+            return Self(self.dropLast())
+        } else {
+            return self
+        }
+    }
+}