Move react-native-implementation.js to index.js

Summary: I am unsure whether there was a reason not to use an `index.js` file from the beginning. It always struck me as confusing and odd to have the main API hidden in a folder somewhere. This changes RN to use the standard `index.js` file that is common in almost all JavaScript packages.

Reviewed By: yungsters, rubennorte

Differential Revision: D17314423

fbshipit-source-id: 10eaf4fddd41e91163de7d10c0879b623dab00d7

.flowconfig

@@ -24,7 +24,7 @@
 [include]
 
 [libs]
-Libraries/react-native/react-native-interface.js
+interface.js
 flow/
 
 [options]
@@ -39,7 +39,7 @@ module.file_ext=.ios.js
 
 munge_underscores=true
 
-module.name_mapper='^react-native$' -> '<PROJECT_ROOT>/Libraries/react-native/react-native-implementation'
+module.name_mapper='^react-native$' -> '<PROJECT_ROOT>/index.js'
 module.name_mapper='^react-native/\(.*\)$' -> '<PROJECT_ROOT>/\1'
 module.name_mapper='^[./a-zA-Z0-9$_-]+\.\(bmp\|gif\|jpg\|jpeg\|png\|psd\|svg\|webp\|m4v\|mov\|mp4\|mpeg\|mpg\|webm\|aac\|aiff\|caf\|m4a\|mp3\|wav\|html\|pdf\)$' -> '<PROJECT_ROOT>/Libraries/Image/RelativeImageStub'
 

.flowconfig.android

@@ -24,7 +24,7 @@
 [include]
 
 [libs]
-Libraries/react-native/react-native-interface.js
+interface.js
 flow/
 
 [options]
@@ -39,7 +39,7 @@ module.file_ext=.android.js
 
 munge_underscores=true
 
-module.name_mapper='^react-native$' -> '<PROJECT_ROOT>/Libraries/react-native/react-native-implementation'
+module.name_mapper='^react-native$' -> '<PROJECT_ROOT>/index.js'
 module.name_mapper='^react-native/\(.*\)$' -> '<PROJECT_ROOT>/\1'
 module.name_mapper='^[./a-zA-Z0-9$_-]+\.\(bmp\|gif\|jpg\|jpeg\|png\|psd\|svg\|webp\|m4v\|mov\|mp4\|mpeg\|mpg\|webm\|aac\|aiff\|caf\|m4a\|mp3\|wav\|html\|pdf\)$' -> '<PROJECT_ROOT>/Libraries/Image/RelativeImageStub'