Provide parity in features between native ant-mojo support and antrun plugin

In particular, we need to make sure classpaths, etc. that are available in the antrun plugin are made available in the ant-mojo support. Going forward, it'd be nice to find a way to use a common module for the two ways of using Ant inside Maven.

The features available in the Antrun Plugin are often confused with those available in the ant-mojo support, so this feature parity is critical to make Ant usage in Maven more intuitive.