[Refactoring]: rename pkg/sql/plan to planner

[aunjgr]

Why do you want to refactor this code?

There are two packages named "plan", pkg/pb/plan and pkg/sql/plan

It causes confusion and inconsistency throughout the codebase

Describe the solution you'd like

Move pkg/sql/plan to pkg/sql/planner and rename the package.

Keep pkg/pb/plan as is.

Describe alternatives you've considered

No response

Additional information

No response