go/vcs: allow go get on github.com/ import paths with Unicode letters

Manually apply same change as CL 41822 did for cmd/go/internal/get,
but for golang.org/x/tools/go/vcs, to help keep them in sync.

Updates golang/go#18660.
Helps golang/go#11490.

Change-Id: I6c7759c073583dea771bc438b70f8c2eb7b5ebfb
Reviewed-on: https://go-review.googlesource.com/42017
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed