Replace requireReactLazily with CJS bundle manipulation

to help prevent modern bundlers from requiring React when used
with `@apollo/client/core` (in other words, without any of
Apollo Client's React components). While this approach works
well for applications that aren't using React, it introduces
problems for bundlers and applications that are using React
(as outlined in #6035 and #6352). There are several different ways
we can address this, and we might do something more substantial
in the future, but for now this commit manipulates Apollo Client's
core CJS bundle at build time, to make the React require optional.

Fixes #6035.
Fixes #6352.

config/rollup.config.js

@@ -63,6 +63,37 @@ function prepareCJS(input, output) {
     },
     plugins: [
       nodeResolve(),
+      // When generating the `dist/core/core.cjs.js` entry point (in
+      // `config/prepareDist.js`), we filter and re-export the exports we
+      // need from the main Apollo Client CJS bundle (to exclude React related
+      // code). This means that consumers of `core.cjs.js` attempt to load the
+      // full AC CJS bundle first (before filtering exports), which then means
+      // the React require in the AC CJS bundle is attempted and not found
+      // (since people using `core.cjs.js` want to use Apollo Client without
+      // React). To address this, we make React an optional require in the CJS
+      // bundle.
+      (() => {
+        const cjsBundle = output.replace(`${distDir}/`, '');
+        return {
+          generateBundle(_option, bundle) {
+            const { code } = bundle[cjsBundle];
+            const regex = /var React = require\('react'\);/;
+            const matches = code.match(regex);
+            if (matches && matches.length === 1) {
+              bundle[cjsBundle].code =
+                code.replace(
+                  regex,
+                  "try { var React = require('react'); } catch (error) {}"
+                );
+            } else {
+              throw new Error(
+                'The CJS bundle could not prepared as a single React ' +
+                'require could not be found.'
+              );
+            }
+          }
+        }
+      })()
     ],
   };
 }

examples/bundling/tree-shaking/rollup-ac3/package.json

@@ -30,8 +30,8 @@
   "dependencies": {
     "@apollo/client": "file:../../../../dist",
     "graphql": "^14.5.8",
-    "react": "^16.13.0",
-    "react-dom": "^16.13.0"
+    "react": "file:../../../../node_modules/react",
+    "react-dom": "file:../../../../node_modules/react-dom"
   },
   "devDependencies": {
     "@babel/preset-react": "^7.0.0",