This makes it easier to make "standard" caddy builds, since you'll only
need to add a single import to get all of Caddy's standard modules.
There is a package for all of Caddy's standard modules (modules/standard)
and a package for the HTTP app's standard modules only
(modules/caddyhttp/standard).
We still need to decide which of these, if not all of them, should be
kept in the standard build. Those which aren't should be moved out of
this repo. See #2780.
