Deprecate cache directory ~/.coursier/cache
This impacts users who used a coursier version released before Nov. 2017. These users should have a ~/.coursier/cache directory on their disk. For these users, this directory should be picked as default cache directory most of the time, per the rules introduced along with the newer cache location.
As of this version, and until 2.0.0 final, running the coursier CLI will print a warning if ~/.coursier/cache is found and used as default location per the rules picking the default cache location. That warning message points to the cache-migration tool, that helps migrate ~/.coursier/cache to the newer cache location.
You might have a ~/.coursier/cache directory, and not get that warning. This happens when you first ran a post Nov. 2017 coursier version on your system, that created the OS-specific cache directory, then ran a pre Nov. 2017 one, that created a ~/.coursier/cache nonetheless. You can run cache-migration too, if you'd like to migrate what's under ~/.coursier/cache to the newer cache.
Optimizations
Optimize some data structures used during resolution, thanks to @jtjeferreira.