Given a lib that is not yet on the repl classpath, make it available by
downloading the library if necessary and adding it to the classloader.
Libs already on the classpath are not updated. Requires a valid parent
DynamicClassLoader.
lib - symbol identifying a library, for Maven: groupId/artifactId
coord - optional map of location information specific to the procurer,
or latest if not supplied
Returns coll of libs loaded, including transitive (or nil if none).
For info on libs, coords, and versions, see:
https://clojure.org/reference/deps_and_cli