[zircon][gn][banjo] Generalize C/C++ generation and fix include paths

The present Banjo invocation through GN have hard-coded logic for Banjo
files in the ddk.protocol.* namespace, in that:

* It uses this namespace to distinguish the "dummy" targets (outside of
  this namespace) that shadow manually-written headers elsewhere in the
  DDK.
* The banjo/c.gni and banjo/cpp.gni sub-implementations also hard-code
  assumptions that the generated files will live in ddk/protocol/*
  subdirectories in the generated output.

This change removes the hard-coded assumption for ddk/protocol/* from
banjo/c.gni and banjo/cpp.gni, as well as laying the groundwork for
supporting an additional set of non-dummy Banjo files in the ddk.hw.*
namespace.

Change-Id: Ie5cf0e2001baffb27bd4f6dbcf38321a69d71dce