Make fetching support more methods, be hookable

[quicklisp]

Right now Quicklisp only supports fetching archives, code, metadata, and other supporting information via HTTP (not even HTTPS is available).

It would be good to directly support fetching local files (e.g. file:/// URLs).

It would also be good to make the fetching mechanism hookable, so any method of retrieving files can be supported as long as there's some code in place to do the work.

For example, while it is unlikely that HTTPS will be directly supported by the client, it should be possible to hook into the fetching system so that, via some extra library or code, support fetching via HTTPS is feasible, whether through some lisp code (like Drakma) or by shelling out to something like curl or wget.

It would also make it possible to support unusual proxy or firewall configurations.

Also potentially can make it easier to support mirroring by e.g. rewriting official URLs into mirror URLs.