Issue title: warning when creating a module in namespace "ceylon" or "java"
 Issue body: The spec reserves these namespaces. If we don't want everyone to go calling their modules "ceylon", we should explicitly discourage them from doing it. (And in the IDE, the New Ceylon Module wizard should give the same warning.)

 Comments: 
 Comment 0: Done.

 Comment 1: It should be possible to disable the warning though. It is annoying to open up the sdk module an get the warnings on module and package files, fully knowing that in this particular case the warning is wrong. 

{'source': 'GithubIssues', 'repo_name_with_owner': 'ceylon/ceylon-spec', 'issue_id': 20600933, 'repo_stars': 108.0}