Fix broken exploration "column added" indicators

[seancolsen]

Fixes #3888
Fixes #3889

Notes

All right sorry for fussing about all the work to fix these. It actually wasn't that much work in the end so I just did it. This PR should get these column counts working correctly — even for cases where the same column is linked to the base table via different join paths. Now the full join path is compared and the count indicators are correct.

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[pavish]

Looks good!