Derive import plugin from Element instead of BuildElement

The import element is not really a build element. The main purpose of the BuildElement class is to allow users to run build-commands, install-commands etc. But, import does not run such commands. Moreover, we already override all the methods provided by BuildElement. So it only makes it confusing to have it derived from BuildElement class when it is not a build element.

So, derive it from the base Element class instead.