…from missing ziphash file

Previously, if an extracted module directory existed in the module
cache, but the corresponding ziphash file did not, if the sum was
missing from go.sum, we would not verify the sum. This caused 'go get'
not to write missing sums. 'go build' in readonly mode (now the
default) checks for missing sums and doesn't attempt to fetch modules
that can't be verified against go.sum.

With this change, when requesting the module directory with
modfetch.DownloadDir, if the ziphash file is missing, the go command
will re-hash the zip without downloading or re-extracting it again.

Note that the go command creates the ziphash file before the module
directory, but another program could remove it separately, and it
might not be present after a crash.

Fixes #44872

Change-Id: I64551e048a3ba17d069de1ec123d5b8b2757543c
Reviewed-on: https://go-review.googlesource.com/c/go/+/298352
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 302a400)
Reviewed-on: https://go-review.googlesource.com/c/go/+/299830