plugin: properly handle recursively defined types

plugin: properly handle recursively defined types

Prevent a crash if the same type in two plugins had a recursive
definition, either by referring to a pointer to itself or a map existing
with the type as a value type (which creates a recursive definition
through the overflow bucket type).

Fixes #19258

Change-Id: Iac1cbda4c5b6e8edd5e6859a4d5da3bad539a9c6
Reviewed-on: https://go-review.googlesource.com/40292
Run-TryBot: Todd Neal <todd@tneal.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>