cmd/doc: search for packages in the two-arg case

cmd/doc: search for packages in the two-arg case

When given one argument, as in

	go doc binary.BigEndian

doc would search for the package, but when given two, as in

	go doc binary BigEndian

it would not. Fix the inconsistency.

Fixes #18697
Fixes #18664

Change-Id: Ib59dc483e8d4f91e6061c77a5ec24d0a50e115f0
Reviewed-on: https://go-review.googlesource.com/59413
Reviewed-by: Aliaksandr Valialkin <valyala@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>