cmd/doc: don't print package clauses on error

cmd/doc: don't print package clauses on error

Everybody was deferring a flush when main already
did that, so drop all that nonsense. (Flush was doing
the package clause stuff.) But then make sure we do
get a package clause when there is correctly no output,
as for an empty package. Do that by triggering a
package clause in allDoc and packageDoc.

Slightly tricky but way less intricate than before.

Fixes #37969.

Change-Id: Ia86828436e6c4ab46e6fdaf2c550047f37f353f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/226998
Reviewed-by: Russ Cox <rsc@golang.org>