cubicweb-vcsfile #2459803 enhance schema and import mecanism to support obsolete extension [resolved]

Changes necessary to the ui should go in another story

prioritynormal
typeenhancement
done in1.11.0
load2.000
load left0.000
closed by#563d6c383637 [hg] add phase support, model level (closes #2459803), #6e417762c21a [hg] obsolete csets entail hidden csets (closes #2459803), #908595737132 [hg] obsoletes relation (closes #2459803)