REVIEW: Split up the internal details of the org.sonatype.nexus.email package.

Not extracting a nexus-email-api yet as this api is pretty nasty, so opting as a first step to move the non-exposed bits into org.sonatype.nexus.internal.email and make the public api not depend on configuration model bits which should be considered internal.
Should be `emailerConfiguration.setUsername(config.getUsername());` ?