Filter down reverse dependencies

[yaahc]

part of https://github.com/rust-lang/crater/issues/440

I don't know how to accurately explain this one, so I will wait for @pietroalbini to come in and fill out a proper issue body.

From our discussion on discord

• filter down reverse dependencies -- easy enough for crates.io, might be hard with the current model for git repos (it'd be hard for git repos because the crater server doesn't store the git repos, but we could just move the reverse dependencies check to the agent and solve it) for actually running crater, not sure if we have capacity for it on our instances at the moment, the projects would have to run crater on their own (and that's expensive)

[yaahc]

@pietroalbini I'm guessing this one fell through the cracks, but when you have time could you still add a mentoring comment explaining how best to impl this feature?

[emilyalbini]

Yeah, totally forgot about this. I'll think it over and write the mentoring instructions sometime in the future.

[hdevalence]

@pietroalbini if it's less work for you, don't worry about writing detailed mentoring instructions -- I'm willing to do the work of reading through the source and figuring stuff out, so just a few pointers would be enough to get started :)

[emilyalbini]

Yeah, I thought about this and I think the only viable option is to move the check to the agents.

A flag filter-reverse-dependencies=lazy_static (bikesheddable) can be added to the experiment creation and to the experiment data structure. That way the agent can receive it and, right after a build started, load the dependency graph and set the result to skipped if it's not a reverse dependency.

Performance won't be great as we'll have to still download all the source code on the agents and run the prepare steps, but with git repositories there isn't much more we can do.