Fix message for when override conflicts with other directives (#2943)

When working with the override directive i received this error (names
changed):
`@override cannot be used on field "myType.myField" on subgraph
"mySubgraph" since "myType.myField" on "mySubgraph" is marked with
directive "@requires"`
But requires was used on "myOtherSubgraph".

I changed the error message to state the correct subgraph and changed
the tests to match the expected behavior.

composition-js/src/__tests__/override.compose.test.ts

@@ -672,7 +672,7 @@ describe("composition involving @override directive", () => {
     expect(result.errors).toBeDefined();
     expect(errors(result)).toContainEqual([
       "OVERRIDE_COLLISION_WITH_ANOTHER_DIRECTIVE",
-      `@override cannot be used on field "T.u" on subgraph "Subgraph1" since "T.u" on "Subgraph1" is marked with directive "@provides"`,
+      `@override cannot be used on field "T.u" on subgraph "Subgraph1" since "T.u" on "Subgraph2" is marked with directive "@provides"`,
     ]);
   });
 
@@ -718,7 +718,7 @@ describe("composition involving @override directive", () => {
     expect(result.errors).toBeDefined();
     expect(errors(result)).toContainEqual([
       "OVERRIDE_COLLISION_WITH_ANOTHER_DIRECTIVE",
-      `@override cannot be used on field "T.u" on subgraph "Subgraph1" since "T.u" on "Subgraph1" is marked with directive "@requires"`,
+      `@override cannot be used on field "T.u" on subgraph "Subgraph1" since "T.u" on "Subgraph2" is marked with directive "@requires"`,
     ]);
   });
 

composition-js/src/merging/merge.ts

@@ -1154,7 +1154,7 @@ class Merger {
          return {
            result: true,
            conflictingDirective: directive,
-           subgraph: subgraphName,
+           subgraph: this.names[fromIdx],
          };
        }
     }