Xcode 16.3 breaks Skip by changing the plugin output folder

[marcprux]

Xcode 16.3 changes the plugin output folder from ${BUILT_PRODUCTS_DIR}/../../../SourcePackages/plugins/ to ${BUILT_PRODUCTS_DIR}/../../../Build/Intermediates.noindex/BuildToolPluginIntermediates/, thereby breaking any Skip app for people who upgrade.

We will be providing guidelines about how to manually fix this issue, but for the time being, we recommend sticking with Xcode 16.2 or lower for your Skip projects.

[marcprux]

This is the second time in recent memory that an undocumented change as stymied the Skip build (see https://forums.swift.org/t/swiftpm-included-with-xcode-16b3-changes-plugin-output-folder-to-destination/73220/3).

[marcprux]

There are three places that will need to be updated to both handle the new path and accommodate previous Xcode versions at the same time:

• The generated Android/settings.gradle.kts for Skip app projects (e.g. https://github.com/skiptools/skipapp-hello/blob/main/Android/settings.gradle.kts)
• The GradleHarness.pluginOutputFolder function that SkipDrive uses to link modules to each other (implemented in https://github.com/skiptools/skip/pull/374/files)
• The SkipGradlePlugin.kt that is used by Gradle to identify the location of the transpiled Kotlin. (implemented in Handle new Xcode 16.3 plugin output folder skip-unit#4)