Use file.path as 'source' option for css.parse

Rework plugins may use the original source path for various reasons, such as providing better error messages.

A concrete example of this is `rework-npm`. It uses the `source` parameter to resolve relative paths. Without it, relative path resolution will only happen for files that are in the root directory.

Since the vinyl object will always have the path at this point, we might as well provide it to rework.