Instead of creating a dummy concept scheme from the file name, we now
require a pre-existing concept scheme's URI to be passed as a --scheme
argument when running an LCSV import from the command line. This matches
with the web UI behavior and makes it easier to integrate downstream
with cubicweb-saem_ref as this application has a custom handling of ARK
identifier which we could assign to the concept scheme previously
created during import.

We move the "Importing Skos dataset" down a bit because we may fail
earlier if the --scheme option is not specified and having this message
displayed is meaningless in such cases.

Related to Extranet #44146518.