[Python] scikit-build-core ignores .pyf files when building Python modules: why?

I maintain a Python module here and I'm having difficulty getting it to build correctly.

The following steps apparently successfully install the module and put a file called geopack_tsyganenko.cpython-313-darwin.so in my site-packages:

git clone https://github.com/johncoxon/tsyganenko
pip install ./tsyganenko


This results in the following in IPython:

In [1]: import geopack_tsyganenko

In [2]: geopack_tsyganenko.igrf_gsw_08?
Signature: geopack_tsyganenko.igrf_gsw_08(*args, **kwargs)
Type: fortran
String form: <fortran function igrf_gsw_08>
Docstring:
igrf_gsw_08(xgsw,ygsw,zgsw,hxgsw,hygsw,hzgsw)

Wrapper for ``igrf_gsw_08``.

Parameters
----------
xgsw : input float
ygsw : input float
zgsw : input float
hxgsw : input float
hygsw : input float
hzgsw : input float


However, geopack.pyf specifies that the hxgsw, hygsw, and hzgsw variables should be outputs, and they are not. This indicates that the build process is not seeing the geopack.pyf file correctly.

On the other hand, if I run:

cd tsyganenko/src/tsyganenko
python -m numpy.f2py -c --f77flags="-w" geopack.pyf geopack.f T96.f T02.f -m geopack_tsyganenko --lower


It creates geopack_tsyganenko.cpython-313-darwin.so in /src/tsyganenko and then if I copy that file manually into my site-packages the following happens in IPython:

In [1]: import geopack_tsyganenko

In [2]: geopack_tsyganenko.igrf_gsw_08?
Signature: geopack_tsyganenko.igrf_gsw_08(*args, **kwargs)
Type: fortran
String form: <fortran function igrf_gsw_08>
Docstring:
hxgsw,hygsw,hzgsw = igrf_gsw_08(xgsw,ygsw,zgsw)

Wrapper for ``igrf_gsw_08``.

Parameters
----------
xgsw : input float
ygsw : input float
zgsw : input float

Returns
-------
hxgsw : float
hygsw : float
hzgsw : float


My question is, how do I need to change my CMakeLists.txt file to get the build process to take account of geopack.pyf and compile correctly? At the moment, CMakeLists.txt looks like this:

cmake_minimum_required(VERSION 3.17.2...3.29)
project(${SKBUILD_PROJECT_NAME} LANGUAGES C Fortran)

find_package(
Python
COMPONENTS Interpreter Development.Module NumPy
REQUIRED)

set(CMAKE_VERBOSE_MAKEFILE ON)
set(CMAKE_Fortran_FLAGS "-w")

# F2PY headers
execute_process(
COMMAND "${PYTHON_EXECUTABLE}" -c
"import numpy.f2py; print(numpy.f2py.get_include())"
OUTPUT_VARIABLE F2PY_INCLUDE_DIR
OUTPUT_STRIP_TRAILING_WHITESPACE)

add_library(fortranobject OBJECT "${F2PY_INCLUDE_DIR}/fortranobject.c")
target_link_libraries(fortranobject PUBLIC Python::NumPy)
target_include_directories(fortranobject PUBLIC "${F2PY_INCLUDE_DIR}")
set_property(TARGET fortranobject PROPERTY POSITION_INDEPENDENT_CODE ON)

# Define variables
set(FORTRAN_PYF_FILE ${CMAKE_CURRENT_SOURCE_DIR}/src/tsyganenko/geopack.pyf)
set(FORTRAN_SRC_FILE ${CMAKE_CURRENT_SOURCE_DIR}/src/tsyganenko/geopack.f)
set(FORTRAN_T96_FILE ${CMAKE_CURRENT_SOURCE_DIR}/src/tsyganenko/T96.f)
set(FORTRAN_T02_FILE ${CMAKE_CURRENT_SOURCE_DIR}/src/tsyganenko/T02.f)
set(MODULE_NAME geopack_tsyganenko)
set(F2PY_MODULE ${CMAKE_CURRENT_BINARY_DIR}/${MODULE_NAME}module.c)
set(F2PY_WRAPPERS ${CMAKE_CURRENT_BINARY_DIR}/${MODULE_NAME}-f2pywrappers.f)

add_custom_command(
OUTPUT ${F2PY_MODULE} ${F2PY_WRAPPERS}
DEPENDS
${FORTRAN_PYF_FILE}
${FORTRAN_SRC_FILE}
${FORTRAN_T96_FILE}
${FORTRAN_T02_FILE}
VERBATIM
COMMAND "${PYTHON_EXECUTABLE}" -m numpy.f2py
"${FORTRAN_PYF_FILE}"
"${FORTRAN_SRC_FILE}"
"${FORTRAN_T96_FILE}"
"${FORTRAN_T02_FILE}"
-m ${MODULE_NAME}
--lower
)

python_add_library(
geopack_tsyganenko MODULE "${F2PY_MODULE}"
"${F2PY_WRAPPERS}"
"${FORTRAN_PYF_FILE}"
"${FORTRAN_SRC_FILE}"
"${FORTRAN_T96_FILE}"
"${FORTRAN_T02_FILE}"
WITH_SOABI)
target_link_libraries(${MODULE_NAME} PRIVATE fortranobject)

install(TARGETS ${MODULE_NAME} DESTINATION .)

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