Update BlockImportChecker

Before, the block import checker did mark imports of the form

  import foo.bar.{baz => _, _}

and

  import foo.bar.{baz => buz, _}

as an error, which is wrong because there is no other way to write
such imports in Scala.
Merge pull request #59 from sschaef/block-import-checker-update

Update BlockImportChecker