Minor SQL build fixes

[shangyian]

Summary

This PR has two fixes to the SQL building:

• The upstream_tables list for built SQL should only include source nodes. Including non-source-nodes leads to issues downstream when we try to search for the physical table.
• When searching the dimensions graph during SQL building, we should use breadth-first and not depth-first search. Otherwise we'll end up performing a more expensive join to pull in a dimension when we could've used the shortest path.

Test Plan

• [ ] PR has an associated issue: #
• [ ] make check passes
• [ ] make test shows 100% unit test coverage

Deployment Plan

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