Auto glob folder patterns in project.json

parent #670 

Supplement to https://github.com/aspnet/KRuntime/pull/727
Instead of changing this code, I would make the implementation of GetSourcePattern do the right thing:

```C#
private IEnumerable<string> GetSourcePattern(Project project, JObject rawProject, string propertyName, string[] defaultPatterns)
{
   return GetSourcePatternCore(rawProject, propertyName, defaultPatterns).Select(p => FolderToPattern(p, project.ProjectDirectory));
}
```