Revert "MNG-5552 made classifier part of MavenProject.artifactMap key"

This reverts commit f35698c790da5329590d1ceba0502c02f942efdf.

There are plugins that rely on the broken behaviour and I don't have the
time to fix this in backwards compatible way right now.

http://markmail.org/message/gjfuofw5gpsr5yxz

Signed-off-by: Igor Fedorenko <ifedorenko@apache.org>