Fix the panic message produced for an interface conversion error to
only say "types from different packages" if they are definitely from
different packges. If they may be from the same package, say "types
from different scopes."

Updates #18911
Fixes #26094

Change-Id: I0cea50ba31007d88e70c067b4680009ede69bab9
Reviewed-on: https://go-review.googlesource.com/123395Reviewed-by: Austin Clements <austin@google.com>