cmd/go/internal/modget: clarify error for 'go get' without arguments

If the current directory doesn't contain a package, 'go get' will say
that without additional detail.

If there were no arguments, errors will start with "go get:" instead
of "go get .:".

Fixes #39080

Change-Id: I47366f2a27bce17bd8b79344ad15b8b934a888c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/234681


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>