Move core sonarkit code to xplat

Summary:
This diff only moves the core of sonarkit to the xplat.

- This diff uses `fb_xplat_cxx_library` as a buck rule in xplat as it is the recommended way
- Updated few imports in the files which were not compiling with new buck function
- Due to the new way of import the OSS would break, so fixed it by changing the `podspec` file

Reviewed By: passy

Differential Revision: D8937414

fbshipit-source-id: 6e12cd049bedb496e7a6820be85b2535e70ba09b

iOS/SonarKit/CppBridge/SonarCppWrapperPlugin.h

@@ -10,7 +10,7 @@
 #endif
 
 #import <Sonar/SonarPlugin.h>
-#import <SonarKit/SonarCppBridgingConnection.h>
+#import <SonarKit/CppBridge/SonarCppBridgingConnection.h>
 #import <SonarKit/SonarPlugin.h>
 
 namespace facebook {