This was flagged by ruff check - if we just want to find out if a
package is available, and don't need to actually import it, we can
use importlib.util.find_spec() to resolve it. This can lead to a
moderate speedup too, since the import might be slow.
This adds two CI runs: a quick one that happens for every pull
request and merge to master, and a longer one that happens daily.
This also adds a new installation group to setup.py because the
`easy` group isn't currently installable, and some of the dependencies
specified there need to be present for the tests to run.
