I'm writing a new module which has a dependency on an external library. In Drupal 7 I would use hook_libraries_info giving the name of the library the required assets and Drupal would search the valid locations, libraries directories, to load these assets when required. Drupal 8 introduces *.libraries.yml and after reading https://www.drupal.org/node/1996238 it seems that module developers must now provide a path, relative or absolute, to the required assets.
In my case I will be expecting site builders to download the external library - where should libraries be located in Drupal 8?