cmd/go/internal/get: disallow non-ASCII unicode letters from import paths

The copy of CheckImportPath in path.go and the regular expression for github
repos in vcsPaths together allow import paths with unicode letters with import
paths. These all come from github repos with non-ASCII unicode letters
with paths in directories. This mainly shows up in GOPATH mode, but could
also show up in Module mode when getting a module in GOPROXY=direct mode.

We expect there to not be any significant affected users of this change--
an investingation of github repos that would produce import paths that
would comply with the copy CheckImportPaths that's being removed, but not
modload.CheckImportPaths only surfaced a handful of cases, all of which
seemed to be small test or demonstation repos. But this CL is being
submitted early in the cycle so that it can be backed out if need be.

Updates #29101

Change-Id: I719df4af5b318e1330e90d8a0bffe5bb8d816f4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/251878


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>