Instead of constructing the importer in init, do it lazily as needed.
This lets us select the importer using a command line flag.
The addition of the command line flag will come in a follow-up CL.

Change-Id: Ieb3a5f01a34fb5bd220a95086daf5d6b37e83bb5
Reviewed-on: https://go-review.googlesource.com/37669Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>