What is the purpose behind putting the network code in libbitcoin under
a separate namespace? Do we plan to split this code off into a
libbitcoin-network module?

The purpose of C++ namespaces is to avoid name conflicts. Unlike Java
where the code is organised in highly recursive namespaces, C++
namespaces are supposed to be flat. It is considered bad practice to use
namespaces to organise code.

If we plan to keep it as part of libbitcoin(-system) then maybe it
should be moved into the top-level libbitcoin namespace.

Take this more as a question rather than a recommendation.