Fix explicit dataset (`FROM` and `FROM NAMED` clauses)

[apicouSP]

Summary of changes

WHAT

FROM or FROM NAMED clause: redefine entirely the query's RDF dataset

When a query (SELECT, CONSTRUCT, ASK, DESCRIBE) is using a FROM or FROM NAMED clause, we redefine entirely the query's RDF dataset Include only the graphs in FROM clauses in the query's default graph Include only the graphs in the FROM NAMED clauses in the query's named graphs

As a consequence, when a user defines a FROM clause in his query but does not define FROM NAMED, then the named graphs are considered empty set. And vice versa: if a user defines a FROM NAMED clause but does not define a FROM clause, then the default graph is considered empty. I added tests specifically for that That's my interpretation of this

Also since the RDF Dataset is entirely redefined when using FROM or FROM NAMED clauses then for those queries the parameter SPARQL_DEFAULT_GRAPH_UNION will be ignored

Load external graphs only if they don't already exist

Try to load external graphs only if they don't already exist in the given ConjunctiveGraph

As a consequence, you don't have to set SPARQL_DEFAULT_GRAPH_UNION to False if all the graphs mentioned in FROM and FROM NAMED clauses already exist in the given ConjunctiveGraph

WHAT it is not

• I didn't change the behavior of triples duplicated in different named graphs when those named graphs are merged into the query's default graph. Would need further discussion
• I didn't change the behavior or the updates (DELETE and INSERT). Since code is independent, I will probably make another PR for that

WHY:

So queries behave closer to W3C spec

Issues:

This PR fixes this issue (confirmed) And also that issue as far as I understand This also solves partially this discussion

Checklist

• [X] Checked that there aren't other open pull requests for the same change.
• [X] Checked that all tests passes .
• [] type checking: black and flake8 do not agree on how to format some lines, I chose to take black's suggestions
• If the change has a potential impact on users of this project:
  • [X] Added or updated tests that fail without the change.
  • [ ] Updated relevant documentation to avoid inaccuracies. Just following W3C spec, which link is already in the docs.
  • [ ] Considered adding additional documentation.
• [X] Considered granting push permissions to the PR branch, so maintainers can fix minor issues and keep your PR up to date.

[apicouSP]

I now use Dataset instead of ConjunctiveGraph as this one is now deprecated. I also fixed style and type errors in new tests. Now black, ruff and mypy should pass. Let me know if you need something to merge this! Sorry I made too much commits.

[coveralls]

coverage: 91.058% (+0.03%) from 91.03% when pulling 8ed095c90a0c6eedf614857a9417fe9145fef65e on apicouSP:fix-explicit-dataset into 0ecc40009ae397c2798c0c08a2d751a1a9d2f8a7 on RDFLib:main.