Fix CMake --find-package mode

[cottsay]

There is a CMake mode which can be used to extract basic compile and linker flags as would be produced by a call to find_package(). Existing ament_cmake code always assumes that we're being invoked under a PROJECT role. This change skips certain project-specific parts of ament_cmake_core and ament_cmake_test which shouldn't be run unless we're actually configuring a CMakeLists.txt project.

The smoke tests added by this change validate that the "extra" CMake functionality provided by these packages continues to be loadable outside of a project context. It does not validate that the defined functions can actually be used, but will serve as a validation that features like CMake --find-package should work.

Docs: https://cmake.org/cmake/help/latest/manual/cmake.1.html#find-package-tool-mode

Expected:

$ cmake --find-package -DNAME=rmw -DCOMPILER_ID=GNU -DLANGUAGE=C -DMODE=COMPILE
-I/opt/ros/rolling/include/rcutils -I/opt/ros/rolling/include/rosidl_dynamic_typesupport -I/opt/ros/rolling/include/rosidl_runtime_c -I/opt/ros/rolling/include/rmw

Actual:

$ cmake --find-package -DNAME=rmw -DCOMPILER_ID=GNU -DLANGUAGE=C -DMODE=COMPILE
CMake Error: Generator implementation error, all generators must specify this->FindMakeProgramFile
CMake Error at /opt/ros/rolling/share/ament_cmake_core/cmake/ament_cmake_package_templates-extras.cmake:17 (message):
  ament_cmake_package_templates: target 'Python3::Interpreter' must exist
Call Stack (most recent call first):
  /opt/ros/rolling/share/ament_cmake_core/cmake/ament_cmake_coreConfig.cmake:41 (include)
  /opt/ros/rolling/share/rmw/cmake/ament_cmake_export_dependencies-extras.cmake:15 (find_package)
  /opt/ros/rolling/share/rmw/cmake/rmwConfig.cmake:41 (include)
  /usr/share/cmake/Modules/CMakeFindPackageMode.cmake:182 (find_package)


rmw not found.
CMake Error: Run 'cmake --help' for all supported options.

Note

This PR changes a bunch of indentation, so it's best to view the changes with whitespace ignored.