What matters during go get -u is not whether there is an import comment
but whether we resolved the path by an HTML <meta> tag.

Fixes #16471.

Change-Id: I6b194a3f73a7962a0170b4d5cf51cfed74e02c00
Reviewed-on: https://go-review.googlesource.com/31658
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>