SIP 46 - avoid duplication of jars in binary distribution

[bishabosha]

currently in 3.5.0-RC1 and 3.5.0-RC2 the binary distribution contains both a lib directory, and a maven2 directory, which duplicate the jars necessary to run all of the commands. (About 40MB)

the classpath for scalac and scaladoc commands are resolved from lib, but the new scala command resolves jars from maven2.

I propose to drop the lib directory, and instead tell scalac and scaladoc commands to resolve from maven2 also.

As an implementation detail, we can explicitly write the classpath to a file, which would also solve issue #20413