[BUG] cimport of editable-installed pxd dependencies does not work #7326

Describe the bug

Say I have a package, mainpkg, that depends on a pxd file, test.pxd in another package, deppkg, like this

from deppkg.test cimport Test

where deppkg/test.pxd is:

cdef class Test:
	int x

If I install deppkg as editable, using pip install -e deppkg, and then try to run

pip install mainpkg --no-build-isolation

(--no-build-isolation is necessary because I need the pxd file at build time), Cython fails to find deppkg/test.pxd with this error

Error compiling Cython file:
------------------------------------------------------------
...
from deppkg.test cimport Test
^
------------------------------------------------------------
mainpkg/test.pyx:1:0: 'deppkg/test.pxd' not found
Compiling ./mainpkg/test.pyx because it changed.
[1/1] Cythonizing ./mainpkg/test.pyx

If I install deppkg without the -e flag, I can build mainpkg.

Code to reproduce the behaviour:

It's all in the repo at https://github.com/cpcloud/cython-editable-bug

Expected behaviour

I would expect the build-time behavior of a dependency to be not dependent on whether a package is installed editably or not.

OS

Linux

Python version

3.14

Cython version

3.2.1

Additional context

No response