Supplying a source map URL doesn't make sense for a file importer, for
two reasons:
1. A canonical `file:` URL is already the recommended source map URL
because it's always available on the local system.
2. Because a FileImporter doesn't know the fully-resolved canonical
URL of the file it loads, it doesn't have any way of determining
what the full source map URL should be anyway.
This was originally part of the legacy API, but I'm bringing it into
the main API so that *all* JS Sass packages provide identifying
information in the same format.
