cmd/compile: move unified IR's reader2 into importer package

cmd/compile: move unified IR's reader2 into importer package

This keeps cmd/compile/internal/importer similar to how
go/internal/gcimporter will work after unified IR support is added in
a subsequent CL.

Change-Id: Id3c000f3a13a54a725602552c6b3191d1affb184
Reviewed-on: https://go-review.googlesource.com/c/go/+/388614
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>