Instead of making a NETTYPE definition dependent on an implementation
module, this change makes it dependent on a pseudo-module for each
specific NETTYPE and makes the respective implementation modules
dependent on it.
This has two advantages:
- one does not need include the whole implementation module to
subscribe to a NETTYPE for testing or to provide an alternative
implementation
- A lot of circular dependencies related to GNRC could be untangled.
E.g. the only reason `gnrc_icmpv6` needs the `gnrc_ipv6` is because it
uses `GNRC_NETTYPE_IPV6` to search for the IPv6 header in an ICMPv6
when demultiplexing an ICMPv6 header.
This change does not resolve these dependencies or include usages where
needed. The only dependency change is the addition of the
pseudo-modules to the implementation modules.
