initial cut, only run jobs for modified packages

[jakemac53]

Ugh, I ran into an issue here that might be a bit hard to reconcile.

This doesn't appear to work well with our idea of "stages". Jobs in later stages depend on all the jobs from previous stages completing, and I think they get skipped if any of those get skipped. But some of them might be for other packages, which were skipped due to not being modified.

We could do the following potentially:

• make this new option be incompatible with merge_stages
• make stages only apply within the same package. So if the first stage for a package completes, all its other stages run, even if some jobs from the earlier stage for different packages failed.
• maybe only apply the previous logic if this option is enabled.

[jakemac53]

I think that the easiest solution here is going to be creating separate workflow files for each package, and using the regular workflow level config for filtering which workflows run. We can investigate that option in a different PR as it will look quite different.

Note that it will be incompatible with the logic to merge jobs across packages, but that should be an OK tradeoff.