add sourceUrl content to source infos for parsed source maps

[vzaidman]

While sourceContent is an optional source map field, it is required for certain behaviors since it's the only field that is used to populate #sourceInfos/#sourceInfoByURL that serves as a source-map-to-source-content mapping for source maps.

This creates issues with legitimate source maps that omit sourceContent and provide source urls instead. e.g. see the description for this bug that I've just fixed on React Native's Metro Hot Reload where the code that checks if source maps are compatible with each other fails to recognize they are not compatible, unless sourceContent is provided, thus adding source maps for files instead of replacing them for files with different source code.

I don't have the context on what's the full correct solution, but essentially, I think #sourceInfos/#sourceInfoByURL should be populated with the contents of source files fetched from sources if sourceContent is missing.

[bmeurer]

Thanks for the patch! Can you please upload the fix via Gerrit, see this part of the Contribution Guide for details.