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/SonarKitTests/SonarCppBridgingTests.mm

@@ -9,7 +9,7 @@
 
 #if FB_SONARKIT_ENABLED
 
-#import <SonarKit/SonarCppWrapperPlugin.h>
+#import <SonarKit/CppBridge/SonarCppWrapperPlugin.h>
 #import <SonarKit/SonarPlugin.h>
 
 using facebook::sonar::SonarCppWrapperPlugin;