Rather than constructing this explicitly to match Node Sass's API, we
construct it with canonical URLs and convert it into the format
expected by the Node Sass API (a mix of paths and URLs) in the
compatibility layer.
In addition to being useful for users of Sass, this will make it
possible for core Sass functions to produce warnings without needing
an explicit reference to the evaluator.
When a stylesheet is imported, the parsed stylesheet object is cached
based on its canonical URL. However, the stylesheet.span.sourceUrl was
based on the text of the import that was used to load that stylesheet.
The idea was to make the source URL in stack traces look nicer, but it
meant that relative URLs could be resolved based on the old importer's
URL before being sent to the new importer, which caused bugs.
Now stylesheet.span.sourceUrl is always the canonical URL of the
stylesheet, and thus safe to cache. We then use the import cache to
convert the canonical URL to a human-friendly URL at the point at
which we generate stack traces.
This also deprecates support for relative canonical URLs. The
semantics of these URLs were always unclear, and with the new change
in import internals the old behavior doesn't make much sense. It's
preserved for backwards-compatibility, but deprecated.
We were resolving URLs relative to the canonical URL rather than the
original URL, which broke importers for which those were different,
like the package importer.
Closes#334
