iOS Codegen: bring back support for defining external libraries in react-native.config.js (#42771)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/42771

This feature was first introduced here https://github.com/facebook/react-native/pull/34580
And then removed here https://github.com/facebook/react-native/pull/41654
The motivation for its removing was that Node resolver should handle all those cases for which `react-native.config.js` was used. But it turns out that it fails for the setup that `react-native-builder-bob` has.
This diff brings back support for defining external libraries in `react-native.config.js`.

Changelog: [iOS][Fixed] - Bring back support for defining external libraries in react-native.config.js

Reviewed By: cipolleschi

Differential Revision: D53267857

fbshipit-source-id: 7625dfe7b4a4651eb60eaec725f94f222a244e30

packages/react-native/scripts/codegen/generate-artifacts-executor.js

@@ -227,6 +227,48 @@ function findExternalLibraries(pkgJson) {
   });
 }
 
+function findLibrariesFromReactNativeConfig(projectRoot) {
+  const rnConfigFileName = 'react-native.config.js';
+
+  console.log(
+    `\n\n[Codegen] >>>>> Searching for codegen-enabled libraries in ${rnConfigFileName}`,
+  );
+
+  const rnConfigFilePath = path.resolve(projectRoot, rnConfigFileName);
+
+  if (!fs.existsSync(rnConfigFilePath)) {
+    return [];
+  }
+  const rnConfig = require(rnConfigFilePath);
+
+  if (rnConfig.dependencies == null) {
+    return [];
+  }
+  return Object.keys(rnConfig.dependencies).flatMap(name => {
+    const dependencyConfig = rnConfig.dependencies[name];
+
+    if (!dependencyConfig.root) {
+      return [];
+    }
+    const codegenConfigFileDir = path.resolve(
+      projectRoot,
+      dependencyConfig.root,
+    );
+    let configFile;
+    try {
+      configFile = readPkgJsonInDirectory(codegenConfigFileDir);
+    } catch {
+      return [];
+    }
+
+    return extractLibrariesFromJSON(
+      configFile,
+      configFile.name,
+      codegenConfigFileDir,
+    );
+  });
+}
+
 function findProjectRootLibraries(pkgJson, projectRoot) {
   console.log('[Codegen] Searching for codegen-enabled libraries in the app.');
 
@@ -447,7 +489,11 @@ function findCodegenEnabledLibraries(pkgJson, projectRoot) {
   if (pkgJsonIncludesGeneratedCode(pkgJson)) {
     return projectLibraries;
   } else {
-    return [...projectLibraries, ...findExternalLibraries(pkgJson)];
+    return [
+      ...projectLibraries,
+      ...findExternalLibraries(pkgJson),
+      ...findLibrariesFromReactNativeConfig(projectRoot),
+    ];
   }
 }