When a plugin uses a relative path for their target's source, this will result in an error:

xcodebuild: error: Could not resolve package dependencies:
  target 'plugin_1' in package 'plugin_1' is outside the package root

    targets: [
        .target(
            name: "plugin_1",
            dependencies: [],
            path: "../Classes",
        )
    ]

This happens because plugins are symlinked in the same directory (introduced in #168932).

This pattern is not part of our migration guide, so I'm not sure why plugin authors are migrating this way. A couple example I found of people doing this:

https://github.com/rishabhsoftwarepvtltd/FlutterSecureVault/blob/5a91e3d1e789920f1863cde360bbd78eea2a1a5c/ios/rspl_secure_vault/Package.swift#L19-L22
https://github.com/Corkscrews/opus_flutter/blob/34b20c9e30601456e51baf1ac77b286969fb5037/opus_flutter_ios/ios/opus_codec_ios/Package.swift#L24