xcodegen: Show absolute path in skipping warning

This would have saved me the time I spent figuring out why something
went wrong with the project generation. In my case, the absolute path
shown by this warning would have been overtly invalid because the tool
was failing to infer the source root directory.

utils/swift-xcodegen/Sources/SwiftXcodeGen/Generator/ProjectSpec.swift

@@ -130,8 +130,9 @@ extension ProjectSpec {
     _ path: RelativePath, for description: String
   ) -> RelativePath? {
     let path = mapKnownPath(path)
-    guard repoRoot.appending(path).exists else {
-      log.warning("Skipping \(description) at '\(path)'; does not exist")
+    let absPath = repoRoot.appending(path)
+    guard absPath.exists else {
+      log.warning("Skipping \(description) at '\(absPath)'; does not exist")
       return nil
     }
     return path