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/wine-host/bridges/common.cpp

@@ -48,7 +48,7 @@ constexpr int extended_max_win32_messages = 8192;
 constexpr unsigned int juce_message_id = WM_USER + 123;
 
 HostBridge::HostBridge(MainContext& main_context,
-                       boost::filesystem::path plugin_path,
+                       ghc::filesystem::path plugin_path,
                        pid_t parent_pid)
     : plugin_path_(plugin_path),
       main_context_(main_context),