We add a --format option to "skos-import" ccplugin command with values
being "rdf" or "lcsv".

The lcsv format import behaves a bit differently than RDF import:

* only on file is accepted,
* a concept scheme is created beforehand to hold concepts to be imported
  from the LCSV file,
* this scheme is eventually dropped in case of import failure or
  interruption
* since lcsv_extentities() accepts a file object, we have to handle file
  opening and closing ourselves, hence the extentities_generator()
  wrapper function.

Closes extranet #37463080.