fix(cmake): unset stale PYTHON_MODULE_DEBUG_POSTFIX and correct USE_PYTHON_INCLUDE_DIR variable (#6086)

When the Python executable changes between CMake runs,
PYTHON_MODULE_DEBUG_POSTFIX was not being unset alongside PYTHON_IS_DEBUG
and PYTHON_MODULE_EXTENSION, allowing a stale postfix to survive interpreter
changes.

In the USE_PYTHON_INCLUDE_DIR block, the elseif branch checked
PYTHON_INCLUDE_DIR (singular) but expanded PYTHON_INCLUDE_DIRS (plural),
always silently resolving to an empty string. Correct it to expand
${PYTHON_INCLUDE_DIR}. Also add a Python3_INCLUDE_DIRS branch so the
option works when pybind11NewTools resolves Python via the Python3
find-package component.

Assisted-by: ClaudeCode:claude-fable-5