RNGP - Fix defaults for PrivateReactExtension

Summary:
When building from source, the PrivateReactExtension is getting no defaults (or missing defaults).
Specifically root should point to ../../ (as the build from source will originate
from `./node_modules/react-native`).

Without that root specified, all the subsequent paths are broken,
specifically, the default being `../` causes the codegen to be searched inside:
```
project/node_modules/node_modules/react-native/codegen
```
which is broken

Changelog:
[Internal] [Changed] - RNGP - Fix defaults for PrivateReactExtension

Reviewed By: cipolleschi

Differential Revision: D43435590

fbshipit-source-id: 2ed5e26c1d63fd808fc2d559ea83d6d39d106ff6

packages/react-native-gradle-plugin/src/main/kotlin/com/facebook/react/internal/PrivateReactExtension.kt

@@ -25,11 +25,32 @@ abstract class PrivateReactExtension @Inject constructor(project: Project) {
 
   private val objects = project.objects
 
-  val root: DirectoryProperty = objects.directoryProperty()
-
-  val reactNativeDir: DirectoryProperty = objects.directoryProperty()
-
-  val nodeExecutableAndArgs: ListProperty<String> = objects.listProperty(String::class.java)
-
-  val codegenDir: DirectoryProperty = objects.directoryProperty()
+  val root: DirectoryProperty =
+      objects
+          .directoryProperty()
+          .convention(
+              // This is the default for the project root if the users hasn't specified anything.
+              // If the project is called "react-native-github"
+              //   - We're inside the Github Repo -> root is defined by RN Tester (so no default
+              // needed)
+              //   - We're inside an includedBuild as we're performing a build from source
+              //     (then we're inside `node_modules/react-native`, so default should be ../../)
+              // If the project is called in any other name
+              //   - We're inside a user project, so inside the ./android folder. Default should be
+              // ../
+              // User can always override this default by setting a `root =` inside the template.
+              if (project.rootProject.name == "react-native-github") {
+                project.rootProject.layout.projectDirectory.dir("../../")
+              } else {
+                project.rootProject.layout.projectDirectory.dir("../")
+              })
+
+  val reactNativeDir: DirectoryProperty =
+      objects.directoryProperty().convention(root.dir("node_modules/react-native"))
+
+  val nodeExecutableAndArgs: ListProperty<String> =
+      objects.listProperty(String::class.java).convention(listOf("node"))
+
+  val codegenDir: DirectoryProperty =
+      objects.directoryProperty().convention(root.dir("node_modules/@react-native/codegen"))
 }