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.