join__field not added to query fields when composeServices is called for one service

Issue Description

The join__field directive is absent from the supergraph if only one subgraph is passed in to composeServices.

This caused an issue for me when onboarding teams to a new registry because initially, when only one schema was onboard, all query fields disappeared when generating a stitched schema via getStitchedSchemaFromSupergraphSdl from @graphql-tools/federation.

The expectation is that join__field will always be added to fields

Link to Reproduction

https://codesandbox.io/p/sandbox/apollo-composition-issue-ky6g83?file=%2Findex.js%3A18%2C5&layout=%257B%2522sidebarPanel%2522%253A%2522EXPLORER%2522%252C%2522rootPanelGroup%2522%253A%257B%2522direction%2522%253A%2522horizontal%2522%252C%2522contentType%2522%253A%2522UNKNOWN%2522%252C%2522type%2522%253A%2522PANEL_GROUP%2522%252C%2522id%2522%253A%2522ROOT_LAYOUT%2522%252C%2522panels%2522%253A%255B%257B%2522type%2522%253A%2522PANEL_GROUP%2522%252C%2522contentType%2522%253A%2522UNKNOWN%2522%252C%2522direction%2522%253A%2522vertical%2522%252C%2522id%2522%253A%2522cloiyyl8w00073b6h667ggidf%2522%252C%2522sizes%2522%253A%255B70%252C30%255D%252C%2522panels%2522%253A%255B%257B%2522type%2522%253A%2522PANEL_GROUP%2522%252C%2522contentType%2522%253A%2522EDITOR%2522%252C%2522direction%2522%253A%2522horizontal%2522%252C%2522id%2522%253A%2522EDITOR%2522%252C%2522panels%2522%253A%255B%257B%2522type%2522%253A%2522PANEL%2522%252C%2522contentType%2522%253A%2522EDITOR%2522%252C%2522id%2522%253A%2522cloiyyl8w00033b6hxow69nu8%2522%257D%255D%252C%2522sizes%2522%253A%255B100%255D%257D%252C%257B%2522type%2522%253A%2522PANEL_GROUP%2522%252C%2522contentType%2522%253A%2522SHELLS%2522%252C%2522direction%2522%253A%2522horizontal%2522%252C%2522id%2522%253A%2522SHELLS%2522%252C%2522panels%2522%253A%255B%257B%2522type%2522%253A%2522PANEL%2522%252C%2522contentType%2522%253A%2522SHELLS%2522%252C%2522id%2522%253A%2522cloiyyl8w00053b6h1u7891v2%2522%257D%255D%252C%2522sizes%2522%253A%255B100%255D%257D%255D%257D%252C%257B%2522type%2522%253A%2522PANEL_GROUP%2522%252C%2522contentType%2522%253A%2522DEVTOOLS%2522%252C%2522direction%2522%253A%2522vertical%2522%252C%2522id%2522%253A%2522DEVTOOLS%2522%252C%2522panels%2522%253A%255B%257B%2522type%2522%253A%2522PANEL%2522%252C%2522contentType%2522%253A%2522DEVTOOLS%2522%252C%2522id%2522%253A%2522cloiyyl8w00063b6hqszg1as8%2522%257D%255D%252C%2522sizes%2522%253A%255B100%255D%257D%255D%252C%2522sizes%2522%253A%255B100%252C0%255D%257D%252C%2522tabbedPanels%2522%253A%257B%2522cloiyyl8w00033b6hxow69nu8%2522%253A%257B%2522id%2522%253A%2522cloiyyl8w00033b6hxow69nu8%2522%252C%2522tabs%2522%253A%255B%257B%2522type%2522%253A%2522FILE%2522%252C%2522filepath%2522%253A%2522%252Findex.js%2522%252C%2522id%2522%253A%2522cloiz136b00oq3b6hcugh78uj%2522%252C%2522mode%2522%253A%2522permanent%2522%252C%2522state%2522%253A%2522IDLE%2522%257D%252C%257B%2522type%2522%253A%2522FILE%2522%252C%2522filepath%2522%253A%2522%252F.codesandbox%252Ftasks.json%2522%252C%2522id%2522%253A%2522cloize2xp03t23b6hbeyg4sve%2522%252C%2522mode%2522%253A%2522permanent%2522%252C%2522state%2522%253A%2522IDLE%2522%257D%255D%252C%2522activeTabId%2522%253A%2522cloiz136b00oq3b6hcugh78uj%2522%257D%252C%2522cloiyyl8w00063b6hqszg1as8%2522%253A%257B%2522id%2522%253A%2522cloiyyl8w00063b6hqszg1as8%2522%252C%2522activeTabId%2522%253A%2522cloiz8tbp01ys3b6hyailhgjh%2522%252C%2522tabs%2522%253A%255B%257B%2522type%2522%253A%2522SANDBOX_INFO%2522%252C%2522id%2522%253A%2522cloiz8tbp01ys3b6hyailhgjh%2522%252C%2522mode%2522%253A%2522permanent%2522%257D%255D%257D%252C%2522cloiyyl8w00053b6h1u7891v2%2522%253A%257B%2522id%2522%253A%2522cloiyyl8w00053b6h1u7891v2%2522%252C%2522activeTabId%2522%253A%2522cloizi2b304vo3b6h6qvs9lqw%2522%252C%2522tabs%2522%253A%255B%257B%2522type%2522%253A%2522TASK_LOG%2522%252C%2522taskId%2522%253A%2522start%2522%252C%2522id%2522%253A%2522cloizi2b304vo3b6h6qvs9lqw%2522%252C%2522mode%2522%253A%2522permanent%2522%257D%255D%257D%257D%252C%2522showDevtools%2522%253Afalse%252C%2522showShells%2522%253Atrue%252C%2522showSidebar%2522%253Atrue%252C%2522sidebarPanelSize%2522%253A15%257D

Reproduction Steps

Call composeServices with a single subgraph

Start at composeServices, then follow the generated supergraph into getStitchedSchemaFromSupergraphSdl; use the linked CodeSandbox reproduction to observe the single-subgraph case. Done means a one-service composition includes join__field on fields and the stitched schema no longer loses query fields.