Preserve true "main" and "browser" fields of package.json modules.

[benjamn]

Previously, when building a JavaScript bundle for the client, if a package.json file had a string-valued "browser" field, we would replace the value of the "main" field of the bundled package.json module with the value of the "browser" field. This trick was important because it allowed an npm package to have a different entry point on the client than it had on the server.

However, that approach became inconsistent if the package.json file was also explicitly imported as a module, because the package.json stub used for module resolution prevented the real contents of package.json from getting bundled, and disagreed with the original package.json module about the value of the "main" field.

To resolve that inconsistency, it seems better to avoid modifying the "main" field of package.json modules, and instead rely on the runtime module system to make sense of the "browser" field, regardless of whether the package.json module is a stub used only for module resolution or contains the full contents of the original package.json file.

The ability to understand "browser" fields of package.json modules was introduced in install@0.8.3: benjamn/install@377d1a3

This is potentially a backwards-incompatible change for developers using this version of ImportScanner and Resolver who have not yet upgraded their modules-runtime package to at least version 0.7.8. The solution is to upgrade modules-runtime, though it would be nice to enforce that better somehow.