Make it easier for users to build from source if needed (#36165)

Summary:
This page is not up to date anymore:
https://reactnative.dev/contributing/how-to-build-from-source

I'm pushing those changes to make it easier to consume our build when building from source.
I'll update the page those change lands.

## Changelog

[Internal] [Changed] - Make it easier for users to build from source if needed

Pull Request resolved: https://github.com/facebook/react-native/pull/36165

Test Plan: If the CI is green, we should be good to merge this

Reviewed By: cipolleschi

Differential Revision: D43303867

Pulled By: cortinico

fbshipit-source-id: c0080b06cbcbf872ee92fcfa82a9f823d5b247f4

package.json

@@ -16,9 +16,11 @@
   },
   "files": [
     "android",
+    "build.gradle.kts",
     "cli.js",
     "flow",
     "flow-typed",
+    "gradle.properties",
     "index.js",
     "interface.js",
     "jest-preset.js",
@@ -61,6 +63,7 @@
     "sdks/.hermesversion",
     "sdks/hermes-engine",
     "sdks/hermesc",
+    "settings.gradle.kts",
     "template.config.js",
     "template",
     "!template/node_modules",

settings.gradle.kts

@@ -13,14 +13,15 @@ pluginManagement {
   }
 }
 
-include(
-    ":ReactAndroid",
-    ":ReactAndroid:hermes-engine",
-    ":ReactAndroid:external-artifacts",
-    ":packages:rn-tester:android:app")
+include(":ReactAndroid", ":ReactAndroid:hermes-engine", ":ReactAndroid:external-artifacts")
 
-// Include this to enable codegen Gradle plugin.
-includeBuild("packages/react-native-gradle-plugin/")
+// If the ./packages folder exists, then we're inside the React Native repository.
+// If not, a users is consuming this project for a build from source.
+if (File("./packages").exists()) {
+  include(":packages:rn-tester:android:app")
+  // Include this to enable codegen Gradle plugin.
+  includeBuild("packages/react-native-gradle-plugin/")
+}
 
 rootProject.name = "react-native-github"