Fix bug where EXTRuntimeExtensions.h and EXTScope.h imports could reference wrong files in cocoapods

To reproduce the bug, create an empty project and pod install with the following podfile:

Podfile
----------------------

platform :ios, '7.0'
import 'Mantle'
import 'ReactiveCocoa'
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Mantle and ReactiveCocoa both include extobjc/, but imports to EXTRuntimeExtensions.h and EXTScope.h aren't fully resolved, such that ReactiveCocoa tries to import EXTRuntimeExtensions.h from Mantle's extobjc/ folder. To fix, import extobjc/EXTRuntimeExtensions.h instead of EXTRuntimeExtensions.h