Must <buildtool_export_depend>ament_cmake_core when using some of the CMake functions

Check out this CMake error from a plain CMake packge https://github.com/ros/angles/pull/32#issuecomment-1015727126

```
CMake Error at /home/sloretz/ws/ros2/install/angles/share/angles/cmake/ament_cmake_export_include_directories-extras.cmake:8 (find_package):
By not providing "Findament_cmake_core.cmake" in CMAKE_MODULE_PATH this
project has asked CMake to find a package configuration file provided by
"ament_cmake_core", but CMake did not find one.

Could not find a package configuration file provided by "ament_cmake_core"
with any of the following names:

ament_cmake_coreConfig.cmake
ament_cmake_core-config.cmake

Add the installation prefix of "ament_cmake_core" to CMAKE_PREFIX_PATH or
set "ament_cmake_core_DIR" to a directory containing one of the above
files. If "ament_cmake_core" provides a separate development package or
SDK, be sure it has been installed.
```

The problem is the CMake code generated as a result of `ament_cmake_export_include_directories()` is find_packaging `ament_cmake_core`. That means any package using `angles` has a buildtool dependency on `ament_cmake_core`, so packages using `ament_cmake_export_include_directories` must [have a `ament_cmake_core