Move ReactCommon's `.../nativemodule/xxx/platform/ios` files to `.../nativemodule/xxx/platform/ios/ReactCommon`

Summary:
To properly setup the Header Search Paths for `use_frameworks!`, we need to move the files that are in the `.../nativemodule/xxx/platform/ios` folder into the `.../nativemodule/xxx/platform/ios/ReactCommon` folder.

We have two folders to migrate: `core` and `sample`.
As a cleanup, `sample` should be extracted from the codebase and moved to RNTester or a library as it can be used as an example to add more examples in the codebase that we ship to our users.

This mimic the same folder structure we have also in `cxx` and `android`

## Changelog
[iOS][Changed] Moved the files from `.../nativemodule/xxx/platform/ios` to `.../nativemodule/xxx/platform/ios/ReactCommon`

Reviewed By: cortinico

Differential Revision: D43082851

fbshipit-source-id: 414426274d9a5ab20cc0e76cdada0c2977264b5f

ReactCommon/react/nativemodule/core/BUCK

@@ -1,5 +1,7 @@
 load("//tools/build_defs/oss:rn_defs.bzl", "ANDROID", "APPLE", "CXX", "FBJNI_TARGET", "get_objc_arc_preprocessor_flags", "get_preprocessor_flags_for_build_mode", "get_static_library_ios_flags", "react_native_target", "react_native_xplat_shared_library_target", "react_native_xplat_target", "rn_xplat_cxx_library", "subdir_glob")
 
+oncall("react_native")
+
 rn_xplat_cxx_library(
     name = "core",
     srcs = glob(
@@ -43,7 +45,7 @@ rn_xplat_cxx_library(
     ],
     ios_exported_headers = subdir_glob(
         [
-            ("platform/ios", "*.h"),
+            ("platform/ios/ReactCommon", "*.h"),
         ],
         prefix = "ReactCommon",
     ),

ReactCommon/react/nativemodule/samples/BUCK

@@ -1,5 +1,7 @@
 load("//tools/build_defs/oss:rn_defs.bzl", "ANDROID", "APPLE", "FBJNI_TARGET", "get_objc_arc_preprocessor_flags", "get_preprocessor_flags_for_build_mode", "get_static_library_ios_flags", "react_native_dep", "react_native_target", "react_native_xplat_target", "rn_android_library", "rn_xplat_cxx_library", "subdir_glob")
 
+oncall("react_native")
+
 rn_xplat_cxx_library(
     name = "samples",
     srcs = glob(
@@ -44,7 +46,7 @@ rn_xplat_cxx_library(
     ],
     ios_exported_headers = subdir_glob(
         [
-            ("platform/ios", "*.h"),
+            ("platform/ios/ReactCommon", "*.h"),
         ],
         prefix = "ReactCommon",
     ),