Treat null importer as noOp if url is not a file: url

[ntkme]

This PR fixes #82.

See sass/sass-spec#1785
See sass/embedded-host-node#128

[nex3]

I think the desirable behavior here, to match the JS API spec as updated in sass/sass#3278, is to set a NoOpImporter only if url isn't a file: URL, and to use a filesystem importer otherwise.

[ntkme]

A relative load's URL is first resolved relative to [[url]], then resolved to a file on disk if it's a file:// URL. If it can't be resolved to a file on disk, it's then passed to [[importers]] and [[loadPaths]].

I think the language I picked matches the current behavior: the js-api (non-embedded) only checks if url is null or not to set it to NoOpImporter or FileSystemImporter. This change and sass/embedded-host-node#124 together makes the embedded host consistent with non-embedded js-api.

Today (non-embedded js-api):

• If url is null -> NoOpImporter
• If url is non-null -> FileSystemImporter

If I understand your proposal correctly:

• If url is null -> NoOpImporter
• If url is non-file url -> NoOpImporter
• If url is file url -> FileSystemImporter

@nex3 Is that what you mean? In that case we also need to update the non-embedded js-api.

[nex3]

@ntkme Looks like this is breaking some legacy importer tests, can you take a look?

[ntkme]

@nex3 Looks like we need sass/embedded-host-node#128. The legacy can resolve from working directory without filename set (meaning no file: url). In other words, it always needs a FileSystemImporter.