Use depends_on.macros as input to state:modified

[jtcohen6]

During state comparison, dbt knows which macros have been modified, but all it does today is log a warning if it finds one:

https://github.com/fishtown-analytics/dbt/blob/f73202734c08b85da739818af2aff74dcbc8638a/core/dbt/graph/selector_methods.py#L435-L445

Why just raise a warning? Well, even though we've long had depends_on.macros for models, we didn't have it for macros—so we didn't have a reliable way to know if a model depends on a macro that depends on a (changed) macro.

For an unrelated purpose, #3272 added a full depends_on.macros for macros, too. We could use this (now reliable) depends_on.macros as an input to state:modified, to select models that call (directly or indirectly) a modified macro.

I think this would look like:

• Find the set of modified macros (we have this today)
• Recursively find the "extended" set of macros that depend on a modified macro
• Find the set of all nodes that depend on any macro in the extended set

I think this could be a simpler way to get at some of the functionality that #2714 described, though the proposal there is much more complex.