@requires field always resolved from same subgraph

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### Issue Description

If one subgraph @requires a field to resolve another field, and that other field is @shareable defined in multiple subgraphs, the query plan seems to arbitrarily pick one subgraph to resolve the shareable field everytime.

This results in a less performant query plan because even if the subgraph that is resolving the query can resolve the @shareable field as well, the query plan still includes the third subgraph just to resolve the shareable field.

Included in the reproduction project I also use @provides to explicitly provide the required field and yet it is still not resolved from the expected subgraph.

I would expect that @shareable would build the most efficient query plan automatically, but especially would expect it when explicitly defining the query plan by using `@provides`.

### Link to Reproduction

https://github.com/joshfoeh-bill/apollo-query-plan

### Reproduction Steps

Reproduction steps included in readme of attached repo.

Start with the README in the linked reproduction repository and reproduce the query plan involving @requires, @shareable, and @provides. Done means the plan selects the subgraph that can resolve the required shareable field when that avoids an unnecessary third-subgraph fetch, including the explicit @provides case.

Written by the indexing model from the issue text.