Fix finding main classes in external jars

[Gedochao]

This fixes finding main classes in external jars, enabling the following syntax:

$ scala-cli run -cp Main.jar --list-main-classes
Main
$ scala-cli run -cp Main.jar
Hello
$ scala-cli --main-class Main -cp Main.jar
Hello
$ ls jarDir
Main.jar
$ scala-cli run -cp jarDir
Hello
$ scala-cli --main-class Main -cp jarDir
Hello

BTW do note that this enables to pass .jar files in a directory, which formerly wasn't being handled.
Also, -cp was added as an alias to -classpath, as part of the SIP backwards compat.