[pybind11] Partially revert the assumption of an automated finder.

It seems pybind11 has its own python find module. The current upstream
head allows using CMake's new FindPython module with the definition
`-DPYBIND11_FINDPYTHON`, but there has been no upstream release with
this feature. In the meantime, we correctly find the vcpkg python3
interpreter and libraries.

ports/pybind11/CONTROL

@@ -3,4 +3,4 @@ Version: 2.5.0
 Port-Version: 1
 Homepage: https://github.com/pybind/pybind11
 Description: pybind11 is a lightweight header-only library that exposes C++ types in Python and vice versa, mainly to create Python bindings of existing C++ code.
-Build-Depends: python3 (windows)
+Build-Depends: python3

ports/pybind11/portfile.cmake

@@ -6,11 +6,28 @@ vcpkg_from_github(
     HEAD_REF master
 )
 
+# pybind master allows opting into the builtin cmake find module with -DPYBIND11_FINDPYTHON-ON.
+# When the upstream port is updated, remove all of this and change the options to just:
+# -DPYBIND11_TEST=OFF -DPYBIND11_FINDPYTHON=ON
+find_program(PYTHON_EXECUTABLE NAMES python python3.8 PATHS "${CURRENT_INSTALLED_DIR}/tools/python3" NO_DEFAULT_PATH)
+find_library(PYTHON_LIBRARY_DEBUG NAMES python38_d python3.8_d PATHS "${CURRENT_INSTALLED_DIR}/debug/lib" NO_DEFAULT_PATH)
+find_library(PYTHON_LIBRARY_RELEASE NAMES python38 python3.8 PATHS "${CURRENT_INSTALLED_DIR}/lib" NO_DEFAULT_PATH)
+
 vcpkg_configure_cmake(
     SOURCE_PATH ${SOURCE_PATH}
     PREFER_NINJA
     OPTIONS
         -DPYBIND11_TEST=OFF
+        -DPYTHON_EXECUTABLE=${PYTHON_EXECUTABLE}
+        -DPYTHONLIBS_FOUND=ON
+        -DPYTHON_INCLUDE_DIRS=${CURRENT_INSTALLED_DIR}/include
+        -DPYTHON_MODULE_EXTENSION=.dll
+    OPTIONS_RELEASE
+        -DPYTHON_IS_DEBUG=OFF
+        -DPYTHON_LIBRARIES=${PYTHON_LIBRARY_DEBUG}
+    OPTIONS_DEBUG
+        -DPYTHON_IS_DEBUG=ON
+        -DPYTHON_LIBRARIES=${PYTHON_LIBRARY_RELEASE}
 )
 
 vcpkg_install_cmake()