Allow custom libraries download location out of plugin data folder

[Ghost-chu]

I would like to be able to customize the dependency download location (outside of the plugin data directory) (e.g. <SERVER_ROOT>/libraries) so that I can reuse downloaded dependencies from SpigotLibraryLoader, avoiding duplicate downloads and wasted disk space. Imagine a large number of plugins all downloading the same dependencies in their own data directories, which would take up a lot of valuable disk space and waste traffic - especially on shared instances.