gddo-server: move code related to pkgsite to pkgsite.go

Pure code in motion.

Change-Id: If883c14b0d7ca1ced8298980111e11cea9614510
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/274696
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
4 files changed