[release-branch.go1.10-security] cmd/go: reject 'get' of paths containing...

[release-branch.go1.10-security] cmd/go: reject 'get' of paths containing leading dots or unsupported characters

On some platforms, directories beginning with dot are treated as
hidden files, and filenames containing unusual characters can be
confusing for users to manipulate (and delete).

Change-Id: I443bdeb98e4de24b8a93a75fb923f4d41052e8f7
Reviewed-on: https://team-review.git.corp.google.com/c/368703


Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>