Fix more issues running in Sublime Text 3

Fixes more issues with running GL Shader Validator in ST3:
1. sublime.packages_path() is blank at startup now due to async load, so permissions check is moved to first actual access
2. Even though view.find() is still supposed to return None when not found, in ST3 it actually returns (-1, -1)
3. By default, Package Control loads ST3 plugins as zipped .sublime-package files, which won't allow the validator executable to run. A blank .no-sublime-package file instructs Package Control to instead load the plugin from a directory, as it does in ST2.