Move extension to renamed Package

[guillep]

RPackage was renamed Package in Pharo 12. Fix extension method

[Ducasse]

I want to move all these extensions to Beautiful comments I have to see how I can do that #684.
Now I cannot load Microdown in P11 because of an extension and I cannot run Pillar in P12 because of OSSprocess
So I will concentrate on P11 because I want to be able to produce book and slides.
For now this is key for me.

[Ducasse]

I will close it and remove all the build* methods. I moved them to BeautifulComments dev (not master because I do not understand the conflict).