Find a suitable concept scheme for encoding/mime type/algorithm relations of a component archive unit

Former constraint was relying on a 'code list version' relation which was setup
on the archive transfer, which a component archive unit doesn't have. To find a
suitable scheme, we may still find the one that is linked to the correct entity
and relation type in the schema (at least until we've several of those, which is
not the case for now).

To do so, add a new `SCHEME_FROM_CONTAINER` structure in xsd2yams (close to
where constraints are defined) so one may easily get the correct query depending
on the container type (ie. wether we are in an archive transfer or an archive
unit).

Also, extend tests to check both case for the 3 aforementionned relations, since
the rql constraint deserves to be tested...