Simplify drop in of 3rd party extensions for Maven core

To enhance a stock Maven distro, 3rd parties can drop additional JARs into the distro's lib directory. If those JARs need to replace default components, the extension needs to be placed first on the classpath to ensure the Plexus container looks up the desired impl. Ensuring that the 3rd party JARs get loaded before the stock Maven JAR requires currently editing the m2.conf used to bootstrap Classworlds. To eliminate this last step, we can support a dedicated ext directory that gets loaded before the default JARs.