Rename Project to buildmetadata-maven-plugin

Since the naming scheme maven-___-plugin for Maven plugins is per convention reserved for plugins written by the Maven team, we have to rename our plugin from maven-buildmetadata-plugin to buildmetadata-maven-plugin.

Although this breaks existing configurations (the artifactId has to be changed), we do not make this a major version number change, since nothing else changes and the version number has to be incremented by the user anyway.