fix classpath issue when files in ~/.m2 get deleted
fix classpath issue when files in ~/.m2 get deleted

checks if all previously downloaded files still exist and
rebuilds everything when anything is missing.

see #77

thheller/shadow-cljs,thheller/shadow-cljs,thheller/shadow-devtools,thheller/shadow-cljs,thheller/shadow-devtools,thheller/shadow-devtools,thheller/shadow-devtools,thheller/shadow-cljs