Fix relativized URIs in source maps (again).

For the options relativize+relativize given to the compiler,
they should be applied before even creating the IR nodes, since
they will be serialized as is without going through the
SourceMapWriter, which previously did this job.

Then, the final relativization w.r.t. the packaged output should
use our Utils.relativize, not the one from URI, so that it can
create relative paths with '..'.