org.apache.maven.toolchain.java.JavaToolchainImpl should be public

org.apache.maven.toolchain.java.JavaToolchainImpl has been created as replacement for the now deprecated org.apache.maven.toolchain.java.DefaultJavaToolChain.

However, the deprecated class was public, the new one isn't. That makes it impossible to follow the deprecation note to use the new class (outside of Maven core). I'm trying to fix this deprecation in Maven Tycho.