Merge: lib: expand packages

I'm currently working on generating README files for packages that don't already have one.
The first step is to create a directory for each package (I call it *expanding* the package).

In this PR, I move all singleton packages to their own directory:
* `lib/a_star.nit` --> `lib/a_star/a_star.nit`
* `lib/x11.nit` --> `lib/x11/x11.nit`
* ...

The `package.ini` file is also moved and renamed.

I used the `nitreadme` bin to do that:

~~~sh
nitreadme --expand lib/
~~~

Next step will be to create the README.md file into the newly created directory.

Pull-Request: #2637
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>

diff --cc lib/libevent/libevent.nit
Simple merge