[Python] Cython *.pxd file cannot use absolute imports for Cython submodule

I am getting an Error compiling Cython file, 'myproject/utils/hdf5.pxd' not found` when trying to build my Cython project in a certain way.

My Cython project is organized this way:

myproject/
setup.cfg
setup.py <-- For installing the Python project

myproject/
__init__.py
reco.pyx
resample.pxd
resample.pyx
setup.py <-- For building the Cython module

utils/
__init__.py
hdf5.pxd
hdf5.pyx
...
setup.py <-- For building the Cython (sub-)module


In myproject/myproject/setup.py (for building the Cython module), I have:

from distutils.core import setup, Extension
from Cython.Build import cythonize

reco = Extension(
name = 'reco',
sources = ['reco.pyx'],
include_dirs = ['.', './utils', *HDF5_DIRS],
...
)

resample = Extension(
name = 'resample',
sources = ['resample.pyx'],
include_dirs = ['.', './utils', *HDF5_DIRS],
....

setup(
name = 'myproject',
packages = ['myproject', 'myproject.utils'],
ext_modules = cythonize([resample, reco]))


Inside resample.pxd:

# cython: language_level=3

import numpy as np
from myproject.utils.hdf5 cimport hid_t, hsize_t, ...
...

cdef inline hid_t write_resampled(...
...


resample.pyx is empty and contains only the Cython language_level declaration.

When I exclude the resample module, this project builds and runs just fine. However, when I try to build it including the resample module, I get several errors related to my imports within resample.pxd; here's one example

python3 setup.py build_ext -if
Compiling resample.pyx because it changed.
[1/1] Cythonizing resample.pyx

Error compiling Cython file:
------------------------------------------------------------
...
import numpy as np
from l4cython.utils.hdf5 cimport hid_t, hsize_t
^
------------------------------------------------------------

./resample.pxd:23:0: 'myproject/utils/hdf5.pxd' not found


This error surprises me because the reco.pyx file contains identical imports, and the reco modules builds just fine.

# Inside reco.pyx

...
from l4cython.utils.hdf5 cimport H5T_STD_U8LE, hid_t, read_hdf5, close_hdf5
...


Further, it seems that if I change the problematic imports in resample.pxd to relative imports, everything works:

# Inside resample.pxd

# Doesn't work:
# from myproject.utils.hdf5 cimport hid_t, hsize_t

# Works:
from utils.hdf5 cimport hid_t, hsize_t


Is there a reason why I can use an absolute cimport from a Cython sub-module in my .pyx file but not in my .pxd file? The C-defined functions in my .pxd file are re-used throughout the program, so I definitely want to have them available for re-use (i.e., not have to copy them to every .pyx file that needs them). However, they depend, in turn, on definitions from other .pxd files.

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