[release-branch.go1.11-security] cmd/go: reject 'get' of paths containing...

[release-branch.go1.11-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: Ia1f5a65b9cff4eeb51cc4dba3ff7c7afabc343f2
Reviewed-on: https://team-review.git.corp.google.com/c/368442


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