Fix classpath detection when using the REPL
Fix classpath detection when using the REPL

This was broken after I stopped appending
`System.getProperty("java.class.path")` to the constructed classpath.
Fixed with a hack, but this is still better than looking at
java.class.path.

dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,som-snytt/dotty,som-snytt/dotty,sjrd/dotty,lampepfl/dotty,lampepfl/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty,lampepfl/dotty,som-snytt/dotty