Extract pom from jar when installing in maven repo

[amichair]

Using the maven-external-dependency-plugin one can install a pom, or a jar, or a jar along with a pom (using the undocumented pomFile configuration, that should be documented). However, most jars built by maven (which is most jars) already have their pom within the jar (under META-INF), so the plugin should use it by default rather than generating a new bare-bones one.