Validate SDK before overwriting (#192)

Running an invalid `sdk` subcommand like `xtool sdk list` (there's no
`list` subcommand) is interpreted as "install the sdk at path `./list`".
We previously unconditionally removed the existing SDK during
installation. By switching the order so that we validate the path first,
we avoid performing a potentially destructive action unless we have
clear intent.

See #186.

Before:

```bash
$ xtool sdk list
Removing existing SDK... # destructive!
Error: Could not read file or directory at path 'list'.
```

After:

```bash
$ xtool sdk list
Error: Could not read file or directory at path 'list'.
  See 'xtool help sdk' for usage.
```

Author: kabiroberai

Sources/XToolSupport/SDKBuilder.swift

@@ -18,7 +18,10 @@ struct SDKBuilder {
         init(path: String) throws {
             var isDir: ObjCBool = false
             guard FileManager.default.fileExists(atPath: path, isDirectory: &isDir) else {
-                throw Console.Error("Could not read file or directory at path '\(path)'")
+                throw Console.Error("""
+                Could not read file or directory at path '\(path)'.
+                  See 'xtool help sdk' for usage.
+                """)
             }
 
             let url = URL(fileURLWithPath: path)

Sources/XToolSupport/SDKCommand.swift

@@ -243,14 +243,15 @@ struct InstallSDKOperation {
         #if os(macOS)
         print("Skipping SDK install; the iOS SDK ships with Xcode on macOS")
         #else
+        // validate input before removing existing SDK
+        let input = try SDKBuilder.Input(path: path)
+        let arch = try ArchSelection.auto.sdkBuilderArch
+
         if let sdk = try await DarwinSDK.current() {
             print("Removing existing SDK...")
             try sdk.remove()
         }
 
-        let input = try SDKBuilder.Input(path: path)
-        let arch = try ArchSelection.auto.sdkBuilderArch
-
         let tempDir = try TemporaryDirectory(name: "DarwinSDKBuild")
         let builder = SDKBuilder(input: input, outputPath: tempDir.url.path, arch: arch)
         let sdkPath = try await builder.buildSDK()