playground/socket: handle multi-file present play snippets
playground/socket: handle multi-file present play snippets

Fixes golang/go#35242

Change-Id: I9621aa0843026ab6331499bcd0ad5ba1e4a21ca5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/204237
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools