[PL-1638] (Compiler) Change Distributed splitter to rely on Independent Graphs at the Operator Level

Summary:
Depends on D4004.  Planner splits the logical plan into the data source side (~= pems) and processor side(= kelvin) using the Dag->IndependentGraphs. If we reuse a non-op variable on both sides of this split, then the IndependentGraphs algorithm will return the entire graph as one entity which is incorrect.
Instead had to make a new Independent Graphs algorithm that only looks at Operators as the actual graph.

Test Plan: old queries work, problematic query added as a new test in logical planner

Reviewers: nserrino, jamesbartlett, #engineering

Reviewed By: nserrino, #engineering

JIRA Issues: PL-1638

Differential Revision: https://phab.corp.pixielabs.ai/D4024

GitOrigin-RevId: b2b3a9955ec17b3d8bd52434fde91b6b754b747d