Replace most uses of Boost.{Filesystem,Process}

With the `ghc::filesystem` dependency from the previous commit. If we
can replace the rest of the Boost.Filesystem dependency then we can get
rid the one nasty runtime dependency we have, and it will make
implementing the chainloading simpler since can reuse more code without
bringing in Boost.

src/plugin/host-process.cpp

@@ -23,7 +23,7 @@
 #include "../common/utils.h"
 
 namespace bp = boost::process;
-namespace fs = boost::filesystem;
+namespace fs = ghc::filesystem;
 
 HostProcess::HostProcess(boost::asio::io_context& io_context,
                          Logger& logger,
@@ -157,7 +157,8 @@ GroupHost::GroupHost(boost::asio::io_context& io_context,
         // because it should run independently of this yabridge instance as
         // it will likely outlive it.
         bp::child group_host =
-            launch_host(host_path_, group_socket_path,
+            // FIXME: Boost.Filesystem conversion
+            launch_host(host_path_, group_socket_path.string(),
                         bp::env = plugin_info.create_host_env());
         group_host.detach();