[dev.typeparams] cmd/compile: don't export/import type parameter indices anymore

types2 now determines type parameter indices lazily, so we don't need
them just as we are importing. We set them in types1 as we are importing
the type param list itself.

type param indices are not strongly needed in types1 - we only use them
in one place which could be rewritten. But I kept them in analogy to
types2 (TypeParam.Index).

Fixes #47451

Change-Id: I30631f95c45a259354eaf7ec5194f71e799eb358
Reviewed-on: https://go-review.googlesource.com/c/go/+/340532


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>