Replace --ignore-if-missing with local file check (#184)

This fixes #181.

Tested this locally by building SDKs. It works for building the SDK for armv7 using my build artifacts:

```
swift run swift-sdk-generator make-linux-sdk --swift-version 6.0.3-RELEASE --host x86_64-unknown-linux-gnu --target armv7-unknown-linux-gnueabihf --target-swift-package-path ~/Downloads/swift-6.0.3-RELEASE-ubuntu-jammy-armv7-install
```

And also building an SDK for aarch64 using official swift.org artifacts:

```
swift run swift-sdk-generator make-linux-sdk --swift-version 6.0.3-RELEASE --host x86_64-unknown-linux-gnu --target aarch64-unknown-linux-gnu
```

Author: xtremekforever

Sources/SwiftSDKGenerator/Generator/SwiftSDKGenerator+Copy.swift

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 import SystemPackage
+import Foundation
 
 extension SwiftSDKGenerator {
   func copyTargetSwiftFromDocker(
@@ -109,16 +110,20 @@ extension SwiftSDKGenerator {
   func copyTargetSwift(from distributionPath: FilePath, sdkDirPath: FilePath) async throws {
     logger.info("Copying Swift core libraries for the target triple into Swift SDK bundle...")
 
-    for (pathWithinPackage, pathWithinSwiftSDK, ignoreIfMissing) in [
+    for (pathWithinPackage, pathWithinSwiftSDK, isOptional) in [
       ("lib/swift", sdkDirPath.appending("usr/lib"), false),
       ("lib/swift_static", sdkDirPath.appending("usr/lib"), false),
       ("lib/clang", sdkDirPath.appending("usr/lib"), true),
       ("include", sdkDirPath.appending("usr"), false),
     ] {
-      try await rsync(
-        from: distributionPath.appending(pathWithinPackage),
-        to: pathWithinSwiftSDK, ignoreIfMissing: ignoreIfMissing
-      )
+      let fromPath = distributionPath.appending(pathWithinPackage)
+
+      if isOptional && !doesFileExist(at: fromPath) {
+        logger.debug("Optional package path ignored since it does not exist", metadata: ["packagePath": .string(fromPath.string)])
+        continue
+      }
+
+      try await rsync(from: fromPath, to: pathWithinSwiftSDK)
     }
   }
 }

Sources/SwiftSDKGenerator/Generator/SwiftSDKGenerator.swift

@@ -158,10 +158,9 @@ public actor SwiftSDKGenerator {
     try Data(contentsOf: URL(fileURLWithPath: path.string))
   }
 
-  func rsync(from source: FilePath, to destination: FilePath, ignoreIfMissing: Bool = false) async throws {
+  func rsync(from source: FilePath, to destination: FilePath) async throws {
     try self.createDirectoryIfNeeded(at: destination)
-    let ignoreMissingArgs = ignoreIfMissing ? "--ignore-missing-args" : ""
-    try await Shell.run("rsync -a \(ignoreMissingArgs) \(source) \(destination)", shouldLogCommands: self.isVerbose)
+    try await Shell.run("rsync -a \(source) \(destination)", shouldLogCommands: self.isVerbose)
   }
 
   func rsyncContents(from source: FilePath, to destination: FilePath) async throws {