Provide a way to force the import type

[fbricon]

The goal is to allow users to select/override the import mechanism. Eg. people with customized .classpath in their gradle project could force the eclipse import over gradle (see #285). Or if the directory contains both build.gradle and pom.xml, users could force a maven import over gradle.

[fbricon]

Turns out forcing an Eclipse import over Gradle is useless, since the Gradfle builder still changes the classpath anyway.
So we'll stick with the build-descriptor-is-the-truth mantra for now