Generate source map data from normalized files

Generate source map data from normalized files

If using `inSourceMap` this fix will ensure the copying of `sourcesContent` is based on potentially normalized `sources` values (https://github.com/mozilla/source-map/blob/master/lib/source-map-consumer.js#L304-L309).

For example `normalize` (https://github.com/mozilla/source-map/blob/master/lib/util.js#L80-L123) will rewrite `./dist/mySource.js` to `dist/mySource.js` in the target `_sources` of the `SourceMapConsumer`. As a result `orig_map.sourceContentFor(source, true);` would return `null` since the orginal `source` was no longer available in the consumer. By using the keys generating from the `SourceMapConsumer.constructor` consistency is ensured.