Since the Go package name is already prefixed to generated ObjC
names, the empty extra prefix is useful. Support that by not reverting
to the default extra prefix, "Go", if -prefix "" is specified.
To avoid file name clashes with the Go header files, add ".objc" to
the ObjC-facing header names.
Change-Id: I559fe60d7474521617f23894af247c6019ff2a21
Reviewed-on: https://go-review.googlesource.com/33954
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
