cmd/compile: fix lazy loading in reader2

Calling reader2.obj fully loads the referenced object, which is
necessary in general; but for reading the package index, we just need
to setup the name->index mapping. This CL adds this, so that lazy
loading works as intended.

Change-Id: Ie51d59e2247d99b46f9dc69fba7ce89e2584b7c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/348011


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>