Fix path for todo data so it's consistent on posix and win32

[renatoi]

posix.relative was returning an incorrect path for Windows (../app/app.js vs app/app.js) which was not consistent with the behavior on posix.

It looks like this is an known issue based on the following issues:

• path.posix.resolve, path.posix.relative return wrong path on Windows nodejs/node#13887
• path.posix.relative returns different results for *nix and Windows versions of node nodejs/node#13683

Since path.relative works correctly, we just need to normalize the path using the slash lib. Also, if the file path is already relative (this happens for ember-template-lint) then we can ignore the relative call, preventing the issue cited above.