Go get in mod-enabled packages lets you do go get "pkg@<hash>" or "pkg@<branch>".
Go internally will switch the hash or branch into a pseudo version.
Go mod download should do the same. The bug lay in the fact that the disk cache
was not being written when Go converted the hash/branch into a pseudo version.

Fixes #27947

Change-Id: I94c29a5c95f69ab18a9cd7a2ecade128047c5e36
GitHub-Last-Rev: 668634b3e70206c6eadabae5969fca1b03093b0d
GitHub-Pull-Request: golang/go#28042
Reviewed-on: https://go-review.googlesource.com/c/140257Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>