gotype: fix default importer for Go 1.8.

Importer "source" does not exist in Go 1.8, so this means that
default usage of gotype is broken in that compiler version.

Change-Id: I517520b0cac7c62a3e213d0647a3d621e8ced58c
Reviewed-on: https://go-review.googlesource.com/40091
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
3 files changed