incremental: changes in independent classes in classpath don't propagate

[ting-yuan]

They are expected to NOT propagate through class dependencies. However, they should propagate through outputs. For example, class Lib, which is obtained by getClassDeclarationByName or getDeclarationsFromPackage, and class Src are used to generated output O.kt. When Lib changes, Src should participate in reprocessing, no matter Src depends (directly or indirectly) on Lib or not.

Currently, the API only allows dependencies of outputs to be source files. An intuitive solution is something like assoicate(decl: KSDeclaration) which requires cooperation with processors.

Although we may introduce some virtual placeholders like AnyChanges, how to know whether an output depends on a class in classpath remains to be investigated, because KSDeclaration.containingFile is null for classes from classpath.