All code relating to a caddytls.Config and setting it up from the
Caddyfile is still intact; only the certificate management-related
code was removed into a separate package.

I don't expect this to build in CI successfully; updating dependencies
and vendor is coming next.

I've also removed the ad-hoc, half-baked storage plugins that we need
to finish making first-class Caddy plugins (they were never documented
anyway). The new certmagic package has a much better storage interface,
and we can finally move toward making a new storage plugin type, but
it shouldn't be configurable in the Caddyfile, I think, since it doesn't
make sense for a Caddy instance to use more than one storage config...

We also have the option of eliminating DNS provider plugins and just
shipping all of lego's DNS providers by using a lego package (the
caddytls/setup.go file has a comment describing how) -- but it doubles
Caddy's binary size by 100% from about 19 MB to around 40 MB...!