Function remap 

pub fn remap<F>(outer: &SourceMap, loader: F) -> SourceMap
where
    F: Fn(&str) -> Option<SourceMap>,

Remap a source map by resolving each source through upstream source maps.

For each source in the outer map, the loader function is called to retrieve the upstream source map. If a source map is returned, mappings are traced through it to the original source. If None is returned, the source is kept as-is.

Range mappings (is_range_mapping) are preserved through composition. The ignore_list from both upstream and outer maps is propagated.

This is equivalent to @ampproject/remapping in the JS ecosystem.