Fix for SELECT * inside SELECT * bug

[robons]

Summary of changes

Fixing use of SELECT * in sub-select within SELECT * parent query as discovered in #1722.

Now when an instance of SELECT * is encountered, the query tree/plan builder now correctly considers the projected variables of any sub-select statements when deciding which variables should be projected out.

• fixes SPARQL Sub-query inside sub-query fails #1722

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Added tests for any changes that have a runtime impact.
• Checked that all tests and type checking passes.
• For changes that have a potential impact on users of this project:
  • (NA) Updated relevant documentation to avoid inaccuracies.
  • (NA) Considered adding additional documentation.
  • (NA) Considered adding an example in ./examples for new features.
  • (NA) Considered updating our changelog (CHANGELOG.md).
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage: 90.66% (+0.004%) from 90.656% when pulling c72cb5f on robons:subquery-in-subquery into fbb8279 on RDFLib:main.

[aucampia]

Thanks for the PR @robons and apologies for the late review. The change and test looks good to me, so I'm going to merge it before Monday if there is no further feedback.