This CL moves all of the logic for wiring up imported declarations
into export.go, so that it can be reused by the indexed importer
code. While here, increase symmetry across routines.

Passes toolstash-check.

Change-Id: I1ccec5c3999522b010e4d04ed56b632fd4d712d9
Reviewed-on: https://go-review.googlesource.com/107621
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>