Custom validation rules API

[stepango]

Validation rules:
Self-validation(name)
Dependencies validation(test, annotationPrecessor, projects)
Content validation

[ikarenkov]

We really need this feature in our project for checking target dependencies which we using. We have TV and Mobile modules and it will be nice to have an ability to crate path validation rule for dependencies. It will falls if mobile target has dependency with path which contains "tv" substring it module path (easiest but not best solution).

[stepango]

Completed on v2 as F-022 (validation framework in forma-core): TargetValidator + ContentRule SPI in plugins/core; Android helpers call pure rules. See docs/forma-core-api.md and the :validation facade.