Preparation for moving the object file reader to its own package.

For #22095

Change-Id: I31fe4a10a2c465f8ea4bf548f40918807e4ec6b5
Reviewed-on: https://go-review.googlesource.com/67314
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>