Fixes google#33 for users not changing generatedFilesBaseDir. Stop documenting
generatedFilesBaseDir since it produces poor results, but keep it
functional with its existing Copy semantics. Fixes google#332

Can't reference project from action

I don't know how to make this trigger a warning, for either Groovy and
Kotlin. Both seem to ignore it.

This fixes `class java.lang.Object cannot be cast to class
com.google.protobuf.gradle.ProtobufPlugin` in "testProjectAndroid
succeeds with configuration cache". The configuration cache is somehow
confusing things, including defeating static typing. Redefining the
variable locally avoids the reference to ProtobufPlugin.