Refactor capnp.pyx to be in lib subdirectory

capnp/includes/schema_cpp.pxd

@@ -5,6 +5,7 @@
 
 from libc.stdint cimport *
 from capnp_cpp cimport DynamicOrphan
+from .capnp.helpers.non_circular cimport reraise_kj_exception
 ctypedef unsigned int uint
 ctypedef uint8_t byte
 ctypedef uint8_t UInt8
@@ -718,9 +719,6 @@ cdef extern from "kj/array.h" namespace " ::kj":
         word* begin()
         size_t size()
 
-cdef extern from "helpers/capabilityHelper.h":
-    void reraise_kj_exception()
-
 cdef extern from "kj/array.h" namespace " ::kj":
     cdef cppclass InputStream:
         void read(void* buffer, size_t bytes) except +reraise_kj_exception