Update iOS Fabric-related files to compile on OSS (#29810)

Summary:
Original PR contents:

This pull request updates the Podspecs and associated build scripts, and some source files so they build on OSS. RNTester now compiles with `fabric_enabled` again.

The following changes have been made:

 * Various spots that were pointing to the old `ReactCommon/fabric` location have now been updated to `ReactCommon/react/renderer`
 * Files that were attempting to use internal FB header `FBRCTFabricComponentsPlugins.h` were changed to use `RCTFabricComponentsPlugins.h`
 * `RCTFabricComponentsPlugins` in OSS was updated to include the `Image` fabric component (thanks tsapeta)
 * Replaced old `generate-rncore.sh` build script with new `generate-rncore.js` script which does not require `flow-node` and uses the `react-native-codegen` API directly, so there is no longer any need for an interim `schema-rncore.json` file.
 * Updated Yoga podspec which wasn't fully synced with changes from the main Yoga repo
 * Updated Fabric podspec with additional needed subspecs

Additions to PR by hramos:
* Replaced use of generate-rncore scripts with the original generate-native-modules-specs.sh script, which is now generate-specs.sh and supports both codegen for Native Modules and Components now (TurboModules/Fabric).
* Codegen now runs at build time as part of the Xcode build pipeline instead of as part of `pod install`. The build script is injected by the FBReactNativeSpec pod, as the pod is part of both Fabric and non-Fabric builds.

[General] [Fixed] - RNTester compiles with `fabric_enabled` again

Pull Request resolved: https://github.com/facebook/react-native/pull/29810

Test Plan:
RNTester now compiles and runs in the simulator again when `fabric_enabled` is set to `true`.

```
cd xplat/js/react-native-github/packages/rn-tester
USE_FABRIC=1 pod install
open RNTesterPods.xcworkspace
```

Reviewed By: fkgozali

Differential Revision: D24058507

Pulled By: hramos

fbshipit-source-id: 8b2ea3694e6cb9aa23f83f087e2995fd4320e2bb

Libraries/FBReactNativeSpec/FBReactNativeSpec.podspec

@@ -20,11 +20,27 @@ end
 
 react_native_path = File.join(__dir__, "..", "..")
 srcs_dir = File.join(__dir__, "..")
-codegen_script_path = File.join(react_native_path, "scripts", "generate-native-modules-specs.sh")
+codegen_script_path = File.join(react_native_path, "scripts", "generate-specs.sh")
 codegen_path = File.join(react_native_path, codegen_path_prefix, "react-native-codegen")
-output_dir = File.join(__dir__, "FBReactNativeSpec")
-generated_files = [File.join(output_dir, "FBReactNativeSpec.h"), File.join(output_dir, "FBReactNativeSpec-generated.mm")]
 codegen_command = "CODEGEN_PATH=#{codegen_path} sh '#{codegen_script_path}' | tee \"${SCRIPT_OUTPUT_FILE_0}\""
+modules_output_dir = File.join(__dir__, "FBReactNativeSpec")
+components_output_dir = File.join(react_native_path, "ReactCommon", "react", "renderer", "components", "rncore")
+generated_filenames = [ "FBReactNativeSpec.h", "FBReactNativeSpec-generated.mm" ]
+generated_files = generated_filenames.map { |filename| File.join(modules_output_dir, filename) }
+
+if ENV['USE_FABRIC'] == '1'
+  components_generated_filenames = [
+    "ComponentDescriptors.h",
+    "EventEmitters.cpp",
+    "EventEmitters.h",
+    "Props.cpp",
+    "Props.h",
+    "RCTComponentViewHelpers.h",
+    "ShadowNodes.cpp",
+    "ShadowNodes.h"
+  ]
+  generated_files = generated_files.concat(components_generated_filenames.map { |filename| File.join(components_output_dir, filename) })
+end
 
 folly_compiler_flags = '-DFOLLY_NO_CONFIG -DFOLLY_MOBILE=1 -DFOLLY_USE_LIBCPP=1 -Wno-comma -Wno-shorten-64-to-32'
 folly_version = '2020.01.13.00'
@@ -40,6 +56,7 @@ Pod::Spec.new do |s|
   s.compiler_flags         = folly_compiler_flags + ' -Wno-nullability-completeness'
   s.source                 = source
   s.source_files           = "**/*.{c,h,m,mm,cpp}"
+  s.exclude_files          = "jni"
   s.header_dir             = "FBReactNativeSpec"
 
   s.pod_target_xcconfig    = {
@@ -55,11 +72,11 @@ Pod::Spec.new do |s|
   s.dependency "React-jsi", version
   s.dependency "ReactCommon/turbomodule/core", version
 
-  s.prepare_command = "mkdir -p #{output_dir} && touch #{generated_files.reduce() { |str, file| str + " " + file }}"
+  s.prepare_command = "mkdir -p #{modules_output_dir} #{components_output_dir} && touch #{generated_files.reduce() { |str, file| str + " " + file }}"
   s.script_phase = {
-    :name => 'Generate Native Modules Code',
+    :name => 'Generate Specs',
     :input_files => [srcs_dir],
-    :output_files => ["$(DERIVED_FILE_DIR)/FBReactNativeSpec-codegen.log"],
+    :output_files => ["$(DERIVED_FILE_DIR)/codegen.log"],
     :script => codegen_command,
     :execution_position => :before_compile
   }