Historically, the legacy importer shim has strongly assumed that all
calls to `canonicalize()` would doubled: once as a URL resolved
relative to the current importer, and again as a URL passed to the
importers list. However, this relies on buggy, non-spec-compliant
behavior in Dart Sass: absolute URLs should never be passed to the
current importer, only to the global list of importers.

This change avoids relying on that behavior by instead disambiguating
relative loads using a custom URL protocol prefix. All canonical URLs
passed to Dart Sass now have the prefix `legacy-importer-` added to
the beginning. This allows us to disambiguate relative loads, and
ignore them for non-`file:` URLs, since relative loads and _only_
relative loads will have schemes beginning with `legacy-importer-`.

To avoid regressing the developer experience, we then strip these
prefixes from all URLs before they're passed to any developer-facing
APIs or displayed to the user.