cmd/go: fix go get -t -u path/... containing vendor directories

cmd/go: fix go get -t -u path/... containing vendor directories

A lot of things had to line up to make this break,
but the caching of download results interacted badly
with vendor directories, "go get -t -u", and wildcard
expansion.

Fixes #18219.

Change-Id: I2676498d2f714eaeb69f399e9ed527640c12e60d
Reviewed-on: https://go-review.googlesource.com/34201
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>