cmd/doc: continue to search when package import fails

Keep searching for a package that is both findable and importable. The
current code would always guarantee that a package was findable but
exited if it was not importable.

Fixes #25478

Change-Id: I237b7dfafb930cae02538c4a2e4d5ce0c1058478
Reviewed-on: https://go-review.googlesource.com/114295


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>