go/doc: move code from playExample to a separate function

go/doc: move code from playExample to a separate function

The playExample function is very long. Move the code that finds
top-level declarations and unresolved identifiers to a separate
function.

In a future CL, we will be improving that function by removing
unused declarations.

Change-Id: I5632012674687f23094b2bc90615daaecb2cf525
Reviewed-on: https://go-review.googlesource.com/c/go/+/401757
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>