go/doc: make examples that depend on top-level decls playable

go/doc: make examples that depend on top-level decls playable

Currently, the following example cannot run in playground:

    func a() {
        fmt.Println("A")
    }

    func ExampleA() {
        a()
    }

This CL solves it.

Fixes #23095

Change-Id: I5a492ff886a743f20cb4ae646e8453bde9c5f0da
Reviewed-on: https://go-review.googlesource.com/83615
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>