Move libbiji source to src

Currently the only need for libbiji (or the core source) is the shell search provider. The source of libbiji is already dispersed in libbiji and src directories. Move it to src directory so it's easier to handle (for the time being).

Currently we are moving around a lot of code. We shall move it to better locations (if needed) once the dust settles. And since this doesn't rewrite git history much, it shouldn't be a big problem.