Revert "Check for trailing "/" in URL, not `path.sep`"

@krisselden points out that `filename` has a backslash on Windows.
https://github.com/broccolijs/broccoli/commit/5ba9ad394880fe8e6f1b2c3e9103a21ca0b06966#commitcomment-6509058

This partially reverts commit 5ba9ad394880fe8e6f1b2c3e9103a21ca0b06966.