Refactor plugin to use non-deprecated Leiningen plugin functionality

[esuomi]

The current method used for implementation is deprecated and may disappear in the future, so in lieu of future proofing this plugin, that of course needs to be resolved.

The most sensible way to do this is to in fact split the Leiningen plugin wrapper to its own project and the core of the plugin to its own. This also opens up the door for alternative build tool implementations.