Add pkgconf-pypi entrypoint (#5552)

- This allows `pkgconf --cflags pybind11` and similar commands to work as expected
  if pkgconf is installed from pypi

setup.py

@@ -144,6 +144,10 @@ with remove_output("pybind11/include", "pybind11/share"):
             stderr=sys.stderr,
         )
 
+    # pkgconf-pypi needs pybind11/share/pkgconfig to be importable
+    Path("pybind11/share/__init__.py").touch()
+    Path("pybind11/share/pkgconfig/__init__.py").touch()
+
     txt = get_and_replace(setup_py, version=version, extra_cmd=extra_cmd)
     code = compile(txt, setup_py, "exec")
     exec(code, {"SDist": SDist})

tests/extra_python_package/test_files.py

@@ -106,6 +106,8 @@ py_files = {
     "commands.py",
     "py.typed",
     "setup_helpers.py",
+    "share/__init__.py",
+    "share/pkgconfig/__init__.py",
 }
 
 headers = main_headers | conduit_headers | detail_headers | eigen_headers | stl_headers

tools/setup_main.py.in

@@ -18,6 +18,7 @@ setup(
         "pybind11.include.pybind11.detail",
         "pybind11.include.pybind11.eigen",
         "pybind11.include.pybind11.stl",
+        "pybind11.share",
         "pybind11.share.cmake.pybind11",
         "pybind11.share.pkgconfig",
     ],
@@ -40,7 +41,10 @@ setup(
         ],
         "pipx.run": [
              "pybind11 = pybind11.__main__:main",
-        ]
+        ],
+        "pkg_config": [
+             "pybind11 = pybind11.share.pkgconfig",
+        ],
     },
     cmdclass=cmdclass
 )