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/common/plugins.cpp

@@ -19,7 +19,7 @@
 #include <fstream>
 #include <sstream>
 
-namespace fs = boost::filesystem;
+namespace fs = ghc::filesystem;
 
 LibArchitecture find_dll_architecture(const fs::path& plugin_path) {
     std::ifstream file(plugin_path, std::ifstream::binary | std::ifstream::in);