nishanths and bradfitz cmd/goimports: support multiple comma-separated imports in -local flag
In cmd/goimports, allow for the -local flag to accept a comma-separated
list of import path prefixes. Also, update the imports package
accordingly to support this.

Fixes golang/go#19188

Change-Id: I083d584df8c3a77532f0f66e9c5d970960180e0d
Reviewed-on: https://go-review.googlesource.com/85397
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Latest commit 059bec9 Dec 23, 2017