utils: update the package API

- remove the package singleton and provide a public constructor
- make the distinction of the source and binary package in the API
using the enum type (PKG_BINARY, PKG_SOURCE)
- add a package instance to GVLE
- add the argument curr_package to gvle modelling plugins
- output files are now generated into the current dir for vle and rvle
and into <current_package>/output into gvle