Move most of the Meson build to subdirectories

We sadly cannot call `shared_library()` and `executable()` in these
subdirectories while still maintaining the same `build/` directory
structure, but this is still much cleaner. All of the other build
artifacts are now also gone from the root of `build/` so it's cleaner
overall.

src/plugin/host-process.cpp

@@ -20,7 +20,7 @@
 #include <boost/process/env.hpp>
 #include <boost/process/start_dir.hpp>
 
-#include "src/common/utils.h"
+#include "../common/utils.h"
 
 namespace bp = boost::process;
 namespace fs = boost::filesystem;