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).

Fixes #29230
Fixes #29231

Change-Id: Ic6f97f577d8fafa83ef62438095a5c7ae022881a
Reviewed-on: https://team-review.git.corp.google.com/c/368507


Reviewed-by: Russ Cox <rsc@google.com>
Reviewed-on: https://go-review.googlesource.com/c/154101


Reviewed-by: Bryan C. Mills <bcmills@google.com>