Deduplicate and add version validation of imports

**Motivation**
With #5026 done, it is possible to improve schema import validation by checking the revision of the imports is consistent.

**Acceptance criteria**
1. A schema/ontology should appear only once in the fat schema even if it is imported several times
2. If schemas and ontologies are imported several times (via intermediate schemas), an error is returned to the user mentioning the candidates and where there were resolved from so that the user can take the appropriate action.

No files or tests are named. Start by tracing the schema and ontology import-validation entry point delivered with #5026, then inspect how indirect imports are collected and resolved. Done means each imported schema or ontology appears once, while inconsistent revisions produce an error listing the candidates and their resolution sources.

Written by the indexing model from the issue text.