python3Packages.pymupdf: propagated mupdf-cxx in such a way that it won't be filtered out

Otherwise, we end up in this situation where this derivation works:

```nix
{ pkgs ? import ./. { } }:

pkgs.runCommand "driver" { buildInputs = [ pkgs.python3Packages.pymupdf ]; } ''
  python -c 'import pymupdf;' > $out
''
```

... but this ostensibly identical derivation fails.

```nix
{ pkgs ? import ./. { } }:

pkgs.runCommand "driver" { buildInputs = [ pkgs.python3.withPackages (ps: [ ps.pymupdf ]; } ''
  python -c 'import pymupdf;' > $out
''
```

What's happening is that the propagated input `mupdf-cxx` doesn't pass the filter check in `requiredPythonModules` as it does not have a Python module as defined in that function.

Now, it does.

Huge thanks to hexa who debugged this with me on Matrix.

Co-authored-by: Martin Weinelt <hexa@darmstadt.ccc.de>

pkgs/development/python-modules/pymupdf/default.nix

@@ -6,6 +6,7 @@
   fetchFromGitHub,
   fetchpatch,
   python,
+  toPythonModule,
 
   # build-system
   libclang,
@@ -39,6 +40,8 @@ let
     enablePython = true;
     python3 = python;
   };
+  mupdf-cxx-lib = toPythonModule (lib.getLib mupdf-cxx);
+  mupdf-cxx-dev = lib.getDev mupdf-cxx;
 in
 buildPythonPackage rec {
   pname = "pymupdf";
@@ -87,7 +90,7 @@ buildPythonPackage rec {
     gumbo
   ];
 
-  propagatedBuildInputs = [ mupdf-cxx ];
+  propagatedBuildInputs = [ mupdf-cxx-lib ];
 
   env = {
     # force using system MuPDF (must be defined in environment and empty)
@@ -95,14 +98,14 @@ buildPythonPackage rec {
     # Setup the name of the package away from the default 'libclang'
     PYMUPDF_SETUP_LIBCLANG = "clang";
     # provide MuPDF paths
-    PYMUPDF_MUPDF_LIB = "${lib.getLib mupdf-cxx}/lib";
-    PYMUPDF_MUPDF_INCLUDE = "${lib.getDev mupdf-cxx}/include";
+    PYMUPDF_MUPDF_LIB = "${mupdf-cxx-lib}/lib";
+    PYMUPDF_MUPDF_INCLUDE = "${mupdf-cxx-dev}/include";
   };
 
   # TODO: manually add mupdf rpath until upstream fixes it
   postInstall = lib.optionalString stdenv.hostPlatform.isDarwin ''
     for lib in */*.so $out/${python.sitePackages}/*/*.so; do
-      install_name_tool -add_rpath ${lib.getLib mupdf-cxx}/lib "$lib"
+      install_name_tool -add_rpath ${mupdf-cxx-lib}/lib "$lib"
     done
   '';