Move the code to invalidate the cached pom-scijava to maven-helper

We will need this functionality a lot. A *lot*. Because we will release
more and more stuff to OSS Sonatype and will require maven-helper to
help us with adjusting things afterwards, for which maven-helper will
need up-to-date cached versions, of course.

Since this code was adjusted from bump-pom-scijava, just use the new
'invalidate-cache' sub-command there, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Teach bump-pom-scijava the --deploy-only option

This is for the modus operandi where somebody pushed a new pom-scijava
to 'master' and wants to leverage the Jenkins job to deploy.

The diff is best viewed with the '-w' option (e.g. "git show -w ...").

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>