Rename 'sources' in global.json to something else (e.g. 'projectSearchPaths')

(based on today's discussion on JabbR)

In global.json the sources key is used to specify paths where the K runtime is looking for projects. This is easily confused with the sources section in project.json and also doesn't make it clear that you should be specifying the folder above project.json there.

Proposal: rename sources to projectSearchPaths in global.json, to clear up the confusion.