Build fails with current Mercury `HEAD`

Job #28803 failed for 14b7aa1b: Mercury devs recently removed the mercury_hl component from their library due to being unused. Since cmake/FindMercury.cmake is not ready for a component to "not being found", it causes problems during the build process.

The fix is simple enough, add the following to the _mercury_find_component macro:

  if(NOT ${_upper_component}_LIBRARY_RELEASE AND
     NOT ${_upper_component}_LIBRARY_DEBUG)
    return()
  endif ()