Use custom package.json props to load src tree

See diff for rationale

webpack.config.js

@@ -43,6 +43,18 @@ module.exports = (env, argv) => ({
     },
 
     resolve: {
+        // We define an alternative import path so we can safely use src/ across the react-sdk
+        // and js-sdk. We already import from src/ where possible to ensure our source maps are
+        // extremely accurate (and because we're capable of compiling the layers manually rather
+        // than relying on partially-mangled output from babel), though we do need to fix the
+        // package level import (stuff like `import {Thing} from "matrix-js-sdk"` for example).
+        // We can't use the aliasing down below to point at src/ because that'll fail to resolve
+        // the package.json for the dependency. Instead, we rely on the package.json of each
+        // layer to have our custom alternate fields to load things in the right order. These are
+        // the defaults of webpack prepended with `matrix_src_`.
+        mainFields: ['matrix_src_browser', 'matrix_src_main', 'browser', 'main'],
+        aliasFields: ['matrix_src_browser', 'browser'],
+
         // We need to ensure we can resolve TS files, but that also means we need to define
         // every single extension we might see, ever.
         extensions: [