internal/derrors: rename httpCodes to codes
internal/derrors: rename httpCodes to codes

httpCodes is renamed to codes, since several codes are not HTTP status
codes.

Change-Id: I1d0adb0e4fa26c9eb88d1fdae83fa22939de242a
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/242884
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite