cmd/go/internal/lockedfile: add a sync.Mutex to lockedfile.Mutex

The compiler (and race detector) don't interpret locking a file as a
synchronization operation, so we add an explicit (and redundant)
sync.Mutex to make that property clear.

The additional synchronization makes it safe to parallelize the tests
in cmd/go/internal/modfetch/coderepo_test.go, which cuts the wall time
of that test by around 50%.

Updates #30550

Change-Id: Ief3479020ebf9e0fee524a4aae5568697727c683
Reviewed-on: https://go-review.googlesource.com/c/go/+/170597


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Jay Conrod <jayconrod@google.com>