This commit replaces the existing jsonformat.PlanRendererOpt type with a new
type with identical semantics, located in the plans package.
We needed to be able to exchange the facts represented by
`jsonformat.PlanRendererOpt` across some package boundaries, but the jsonformat
package is implicated in too many dependency chains to be safe for that purpose!
So, we had to make a new one. The plans package seems safe to import from all
the places that must emit or accept this info, and already contains plans.Mode,
which is effectively a sibling of this type.
0df3c143bb