Java 9 and migrator discovery failure

[mattford63]

https://github.com/juxt/joplin/blob/9a16a8b4b35a2d1fef0d3c6901442ee07b0f90ef/joplin.core/src/joplin/core.clj#L94

Swithing to Java 9 stopped Migrators from working :-(

Our migrators are on classpath but in J9

(clojure.java.classpath/classpath-directories) returns ()

whereas in J8 it returns something like

(#object[java.io.File 0x24581963 "/home/matt/src/github/mastodonc/kixi.search/test"]
 #object[java.io.File 0x59969d1f "/home/matt/src/github/mastodonc/kixi.search/dev"]
 #object[java.io.File 0x7e3ac75f "/home/matt/src/github/mastodonc/kixi.search/src"]
 #object[java.io.File 0x3fe13f9f "/home/matt/src/github/mastodonc/kixi.search/resources"]
 #object[java.io.File 0x5b16aa01 "/home/matt/src/github/mastodonc/kixi.search/target/classes"])```