cannot create archetype for existing project without pre-existing pom

Unlike 1.0, the 2.0-alpha-2 version will not allow users to create a new archetype (e.g. a new pom file) for an existing project. In DefaultFilesetArchetype.java, If the archetype is 'partial', and the project directory already exists, then the pom must already exist or an exception is thrown. If the archetype is not partial, and the project directory already exists, then a ProjectDirectoryExists exception is thrown.