Here is an example code. The package_info.txt file will contain the package name but without the extension and path.
# Set packaging path
if (NOT CPACK_OUTPUT_FILE_PREFIX)
set(CPACK_OUTPUT_FILE_PREFIX "${PROJECT_BINARY_DIR}")
endif()
if (NOT CPACK_PACKAGE_DIRECTORY)
set(CPACK_PACKAGE_DIRECTORY "${PROJECT_BINARY_DIR}")
endif()
# Set package file name
set (CPACK_PACKAGE_FILE_NAME "${CPACK_PACKAGE_NAME}-${CPACK_PACKAGE_VERSION}-${CPACK_SYSTEM_NAME}")
if (CPACK_PACKAGE_ARCHITECTURE)
set(CPACK_PACKAGE_FILE_NAME "${CPACK_PACKAGE_FILE_NAME}-${CPACK_PACKAGE_ARCHITECTURE}")
endif()
message(STATUS "CPACK_PACKAGE_FILE_NAME: " ${CPACK_PACKAGE_FILE_NAME})
file(WRITE "${PROJECT_BINARY_DIR}/package_info.txt" "${CPACK_PACKAGE_FILE_NAME}")