Cython: error while importing a .pxd file

I am creating a function to solve differential equations. The structure of my package is the following:

package
 |- integrate
     |- signature.pxd
     |- rk4.pyx
     |- euler.pyx

signature.pxd contains the template of the function that has to be defined and given in input to rk4.pyx and euler.pyx.

The content of signature.pxd is:

import numpy as np
cimport numpy as np

ctypedef np.float64_t FLOAT

cdef class ProblemOde:
    cpdef np.ndarray[FLOAT, ndim=1] evaluate(self, double t, np.ndarray[FLOAT, ndim=1] y):
        return np.ndarray([])

My problem is that I am not able to import it in rk4.pyx and euler.pyx. I cannot use neither absolute or relative import and if I write:

cimport signature
from signature cimport ProblemOde

I get the error: 'signature.pxd' not found

What am I missing? How can I do that?

Thanks a lot!

EDIT: here the setup.py

# built-in/generic imports
import os
from distutils.core import setup

# third-party imports
from Cython.Build import cythonize
import numpy as np

# list of packages eopack
__module_dir__ = os.path.abspath(os.path.dirname(__file__))
packages_eopack = [os.path.join(__module_dir__, 'package', 'integrate', 'rk4.pyx'),
                   os.path.join(__module_dir__, 'package', 'integrate', 'euler.pyx')]

# compiler directives
directives = {'language_level': '3',
              'always_allow_keywords': True}

# compile
setup(ext_modules=cythonize(packages_eopack, compiler_directives=directives, annotate=True),
      include_dirs=[np.get_include()])