Add exclude_dependents to the exclude block

[yhakbar]

Describe the enhancement

In addition to allowing dependencies to be excluded from the run queue when users set exclude_dependencies to true in the exclude block, allow dependents to be excluded when a unit sets exclude_dependents to true.

This should be a recursive exclusion, so all descendant dependents should be excluded in addition to the unit being excluded when exclude_dependents is true.

Additional context

A complicating factor for this feature is the fact that units might not necessarily have context for their dependents when being parsed.

We need to make sure that dependents can be identified before a unit can exclude its dependents.

RFC Not Needed

• I have evaluated the complexity of this enhancement, and I believe it does not require an RFC.