Break Circular Dependency between React-Codegen and React-Fabric (#36210)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/36210

One of the circular dependencies we have in OSS was between React-Codegen and React-Fabric.

React-Codegen generates component which has to depends on React-Fabric because they need to use the files contained in the `react/renderer/view` folder.

React-Fabric contains some components that depends on RNCore, which was generated inside the React-Codegen folder.

This change generates the RNCore components inside the `ReactCommon/react/renderer/components/rncore` folder, breaking the dependency as `rncore` folder is now contained by React-Fabric itself.

**Fun Fact:** That's how it always should have been. There was already a line in the `.gitignore` to exclude the content of `ReactCommon/react/renderer/components/rncore` folder. I guess that with some of the refactoring/previous projects on Codegen, this requirements has slipped.

## Changelog:
[iOS][Breaking] -  generates RNCore components inside the ReactCommon folder and create a new pod for platform-specific ImageManager classes

Reviewed By: sammy-SC, dmytrorykun

Differential Revision: D43304641

fbshipit-source-id: ebb5033ce73dbcd03f880c3e204511fdce04b816

ReactCommon/React-rncore.podspec

@@ -37,5 +37,6 @@ Pod::Spec.new do |s|
     :js_srcs_dir => "#{react_native_path}/Libraries",
     :library_name => "rncore",
     :library_type => "components",
+    :output_dir => "#{react_native_path}/../ReactCommon"
   })
 end