cmd/go: validate path in mod init path

cmd/go: validate path in mod init path

When mod init with given module path, validate that module path is a
valid import path.

Note that module.CheckImportPath is used, because module.CheckPath
verifies that module path is something that "go get" can fetch, which is
strictly stronger condition than "a valid module path".

Updates #28389
Fixes #32644

Change-Id: Ia60f218dd7d79186f87be723c28a96d6cb63017e
Reviewed-on: https://go-review.googlesource.com/c/go/+/182560
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>