Gradle plugin doesn't apply compiler plugins to non-main variants

[RBusarow]

If Tangle is added to a module via the plugin, the compilers are added using the anvil (main) configuration only and the *api dependencies are added via implementation.

This means code in the androidTest srcDir can reference Tangle APIs, but by default nothing will be generated for the annotations.

Since Anvil's plugin automatically adds the compiler plugin for each variant, Tangle probably should as well.