The build currently uses the old maven support in gradle. This commit
switches to use the newer maven-publish plugin. This will allow future
changes, for example, easily publishing to artifactory.
An additional part of this change makes publishing of build-tools part
of the normal publishing, instead of requiring a separate upload step
from within buildSrc. That also sets us up for a follow up to enable
precomit checks on the buildSrc code itself.
Gradle defaults to tgz extension when tar is compressed. This changes
the tar distribution back to tar.gz. Note that this also means the maven
packaging type is now tar.gz.
