Require modules to be listed in package.cnf

[uliska]

Currently packages are exclusively determined by there existence in the file path. Requiring them to be listed in package.cnf seems a little bit redundant but would have the following advantages:

• public documentation - anybody can read package.cnf and see what is present
• introspection - oll-core will reject \loadModule requests already at this stage. Frescobaldi can use it to display information
• ("hiding" of not-yet-ready modules)