[DF] SchemaError on nested RexCall queries

[jdye64]

Running PyTest test_compatibility::test_agg_min_min results in the following error that is not expected.

RuntimeError: SchemaError(FieldNotFound { qualifier: Some("a"), name: "e", valid_fields: Some(["a.a", "a.b", "MIN(a.c)", "MAX(a.c)", "MIN(a.d)", "MAX(a.d)", "MIN(a.e)", "MAX(a.e)", "MIN(a.f)", "MAX(a.f)", "MIN(a.g)", "MAX(a.g)", "MIN(a.a + a.e)", "MIN(a.a)"]) })