Query plan `alias` field is never populated

Doing a search of the code indicates that https://github.com/apollographql/federation/blob/ffe0687ba82d0faf44cb38fbe2c8c3067054dc2d/query-planner-js/src/QueryPlan.ts#L51 is never being written to.

It is read here: https://github.com/apollographql/federation/blob/ffe0687ba82d0faf44cb38fbe2c8c3067054dc2d/query-planner-js/src/QueryPlan.ts#L69

And a candidate for population is here: https://github.com/apollographql/federation/blob/ffe0687ba82d0faf44cb38fbe2c8c3067054dc2d/query-planner-js/src/QueryPlan.ts#L96

It looks like there are no tests that exercise the `alias` field.

Start in query-planner-js/src/QueryPlan.ts at the alias field around lines 51, 69, and 96, tracing how query plans are constructed and read. Add test coverage for the alias field, confirming it is populated and available where it is consumed.

Written by the indexing model from the issue text.