Autoload asdf with require

Describe the bug Loading some contribs like sprof require asdf to be loaded. This is a bit annoying. Let's load asdf automatically when require is called if asdf isn't already loaded.

We don't need to do this for defsystem because no contribs use defsystem to load them.

This will also allow us to simplify the contents of modules.lisp so we don't have to define all the modules. Also, if we add asd files for all the existing cmucl contribs, we can remove the cmu-contribs file and simplify loading for these as well. You can just require the contrib instead of requiring cmu-contribs first. Everything then becomes consistent.