Warn when tasks are modifying their inputs

We should process methods in `Task` implementations annotated with `@TaskAction` and methods transitively called from there. These methods should not call any setters on external types used as property types for said tasks.

No files or tests are named in the issue. Start by locating Task implementations and @TaskAction entry points, then trace transitively called methods and identify setters on external property types; done means the analyzer warns for those calls.

Written by the indexing model from the issue text.