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This may cause problem because absolute_url may embed changing part such as
entity's name or title. So, add a `canonical_uri` class method to the
RDFGraphGenerator which by default returns `<base url>/eid` (this is internally
used in cubicweb, it sounds reasonnable that this url scheme will always be
available).

Closes #15922107

In case where we import several files or alike, we may not want to rebuild it
at each round, because it may be costly if eg. the massive store is used (all
indexes may then be removed).

Remove case of a 'sed-2' language coming from the seda cube, add None (ie no language specified) instead

not sure it has passed once :/

usually to test data file validity.

Code adapted and backported from the SEDA cube.

Closes #14170765

CubicWeb is installed as a dependency of the cube itself.

Closes #13211250.

Similarly as for Concept, as done in 67672a4dd0ef.