replace expensive search method with efficient bottom-to-top one

[NeKJ]

currently the method that searches the .js files for the sourceMap directive is costly and that contributes a lot with largish dependencies to the final build time of a project.

This is a different approach to conduct the same search with the same results (passes all the issued tests) that is efficient for larger dependencies. i.e. in our project the source-map-loader took ~15 seconds and with this takes only 3-4ms.

[jsf-clabot]

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