- Rename conformance from XcodeProjectPlugin to XcodeBuildToolPlugin
  (the correct protocol exported by the XcodeProjectPlugin module)
- Bump swift-tools-version to 6.1 so Target.directoryURL is available
  (it's @available(_PackageDescription 6.1) in PackagePlugin)
- Replace all remaining deprecated URL.path with path(percentEncoded:false)
  in findConfig for clean zero-warning compile