Behavior of `R_lib` and `lib_path` under local config `CONF`

Currently `DSC::R_lib` only checks & installs R libraries without loading them automatically, and `DSC::lib_path` only provide search path for scripts to source without tracking changes. This design makes sense when they are global to all modules. When under module specific `@CONF`, however, we may want to additionally auto-load R library and keep track of file changes under `lib_path`. 