Rename debugtilesource.js to tiledebugsource.js 

This PR fixes #2318 by just renaming the debugtilesource.js file. I went the easy path and did not attempt to do more renaming. More renaming can be done with more issues and PRs.

Please review.

Setting this to the v3.0.0-gamma.4 milestone (which I want to release today).

Closes #2318.