path_relative_to_include() helper should be OS agnostic

[brikis98]

The path_relative_to_include() returns the path between a root and a child .terragrunt file. This is typically used to set the state_file_id and key settings. The problem is that on *nix systems, this path will contain forward slashes, whereas on Windows, for the same path, it will contain backslashes. Therefore, the locking and state file storage won't work properly if team members use different operating systems.

path_relative_to_include should probably enforce forward slashes on all OS's.

[brikis98]

This was fixed by #100.