gddo-server: split main.go into two files

Pure code in motion.

Change-Id: Idd7e08bac2a1fdb9e2c79ca9a126becf30ea4235
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/285835
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>