Allow injecting tasks as dependents

[arcivanov]

Currently when you add a @task you're limited in flexibility in where you can add it.

Unless the new task follows core plugin graph and extends a particular existing task or is invoked as a top-level task, there is no way to do this:

• Given a graph of B->A and top-level B I want C to be dependent on A and become a dependency of B.

This would allow 3rd party plugins to augment the build graph (such as header check plugin) without the user having to add top-level tasks. I.e. pyb publish with an injection of a header check plugin will result in a header check plugin executing a task without having to specify pyb check_headers publish.

All circularity/resolution check enforcements should, naturally, stay in force.

A new construct such as @dependent("task_a", "task_b", optional("task_c", "task_d")) will be introduced.

[arcivanov]

@mriehl review please

[mriehl]

Neat idea, bear with me for a few hours. I'll review today.

[mriehl]

I'd change the name though, I think @needed_by('task_a') is more readable. Very much 👍 on the general idea, this would improve the dependency capabilities for sure.

[arcivanov]

I was thinking of it as:

@task
@depends("task_a", optional("task_b"))
@dependent("task_c", optional("task_d"))
def new_plugin_task():
     pass

I.e. create depends / dependent cadence.