Improve fragment reuse code (#2497)

The existing code was only able to reuse named fragments in a small
subset of cases (essentially when the fragment was matched by a full
sub-selection). This patch makes the code much more able to reuse
fragments by allowing to match sub-selections, handle intersecting
fragments, etc...

.changeset/rich-kings-obey.md

@@ -0,0 +1,11 @@
+---
+"@apollo/query-planner": patch
+"@apollo/federation-internals": patch
+"@apollo/gateway": patch
+---
+
+Improves reuse of named fragments in subgraph fetches. When a question has named fragments, the code tries to reuse
+those fragment in subgraph fetches is those can apply (so when the fragment is fully queried in a single subgraph fetch).
+However, the existing was only able to reuse those fragment in a small subset of cases. This change makes it much more
+likely that _if_ a fragment can be reused, it will be.
+

gateway-js/src/__tests__/integration/requires.test.ts

@@ -218,13 +218,13 @@ it('collapses nested requires with user-defined fragments', async () => {
           {
             user {
               __typename
-              id
               preferences {
                 favorites {
-                  animal
                   color
+                  animal
                 }
               }
+              id
             }
           }
         },

gateway-js/src/__tests__/integration/value-types.test.ts

@@ -98,33 +98,30 @@ describe('value types', () => {
                     reviews {
                       metadata {
                         __typename
-                        ... on KeyValue {
-                          key
-                          value
-                        }
-                        ... on Error {
-                          code
-                          message
-                        }
+                        ...Metadata
                       }
                     }
                   }
                   ... on Furniture {
                     reviews {
                       metadata {
                         __typename
-                        ... on KeyValue {
-                          key
-                          value
-                        }
-                        ... on Error {
-                          code
-                          message
-                        }
+                        ...Metadata
                       }
                     }
                   }
                 }
+                
+                fragment Metadata on MetadataOrError {
+                  ... on KeyValue {
+                    key
+                    value
+                  }
+                  ... on Error {
+                    code
+                    message
+                  }
+                }
               },
             },
             Flatten(path: "topProducts.@") {