Move `extra::net_*` to `extra::net::*` properly.

Where * = tcp, ip, url.

Formerly, extra::net::* were aliases of extra::net_*, but were the
recommended path to use. Thus, the documentation talked of the `net_*`
modules while everything else was written expecting `net::*`.

This moves the actual modules so that `extra::net::*` is the actual
location of the modules.

This will naturally break any code which used `extra::net_*` directly.
They should be altered to use `extra::net::*` (which has been the
documented way of doing things for some time).

This ensures that there is one, and only one, obvious way of doing
things.