Add imported URIs getter to ImportCache

[irundaia]

Hi,

I was looking to implement a feature in dart-sass-embedded, to expose all imported URIs (see sass/embedded-protocol#62).

However, to do this using the current api's, I'd have to first setup a StylesheetGraph including an ImportCache, then use the graph to load the to-be-compiled file, and then compile the file as normal.
I think it would be easier, to expose a getter on the ImportCache class to expose all imported canonical URIs.

[nex3]

We already track similar information in _EvaluateVisitor._includedFiles. Although this is specifically made to match Node Sass's API and so only handles filesystem paths, it could be made to handle URLs instead and then converted to filesystem paths in node.dart.

The real question is how we expose this from the public Dart API. The public compile* methods all return strings, which are difficult to add extra information to. Ideally we'd return a CompileResult like we do for the internal compile* methods, but that would be a breaking change at this point. It's probably easiest to just do the same thing we do for source maps, and have users pass in a callback that gets passed the URLs.

[nex3]

Thanks for the contribution! I've ended up going a different direction in #1398, but I really appreciate you pitching in!