Ability to read and use the prefixes used in SHACL SPARQL target definitions

[tfrancart]

we need to be able to read this:

med:S a sh:NodeShape ;
  sh:target [
    sh:select """PREFIX med: <http://data.esante.gouv.fr/ansm/medicament/>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT $this WHERE { $this rdfs:subClassOf med:SpecialitePharmaceutique }""";
  ] ;