SI-5082 Cycle avoidance between case companions

We can synthesize the case companion unapply without forcing
the info of the case class, by looking at the parameters in
the `ClassDef` tree, rather than at `sym.caseFieldAccessors`.

Access to non-public case class fields routed through the
already-renamed case accessor methods. The renamings are
conveyed via a back-channel (a per-run map, `renamedCaseAccessors`),
rather than via the types to give us enough slack to avoid
the cycle.

This change may allow us to resurrect the case class ProductN
parentage, which was trialled but abandoned in the lead up
to 2.10.0.