@requires field always resolved from same subgraph

[joshfoeh-bill]

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.