[release-branch.go1.13] cmd/go/internal/get: avoid panic in metaImportsForPrefix if web.Get fails

Updates #29591
Updates #34049
Fixes #34081

Change-Id: I817b83ee2d0ca6d01ec64998f14bc4f32e365d66
Reviewed-on: https://go-review.googlesource.com/c/go/+/193259


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
(cherry picked from commit 1eab1aa6ba6c3f4d6f084751bca9a065707c3085)
Reviewed-on: https://go-review.googlesource.com/c/go/+/193264


Reviewed-by: Jay Conrod <jayconrod@google.com>