fix ImportData.ImportType use of sealed

[nbauernfeind]

The compiler says not all types are covered for this piece of code:

private[this] def onImportData(id: ImportData): Unit = {
  id.importType match {
    case ImportData.Type => onImportDataType(id)
    case ImportData.Term => onImportDataTerm(id)
    case ImportData.TermType =>
      onImportDataType(id)
      onImportDataTerm(id)
  }
}

I believe, that this is just simply from an incorrect usage of Scala's sealed.

[nbauernfeind]

@alexarchambault I would be curious what you think of this PR as well =). Thank you in advance!