cmd/go: rename doc.go to alldocs.go in preparation for "go doc"

Also rename and update mkdoc.sh to mkalldocs.sh

Change-Id: Ief3673c22d45624e173fc65ee279cea324da03b5
Reviewed-on: https://go-review.googlesource.com/9226


Reviewed-by: Russ Cox <rsc@golang.org>