parallel routes: fix duplicate dev warning (#47317)

- there's a warning that happens currently when compiling parallel
routes in dev because they all resolve to the same pathname and their
identity function is all the same
- I'm repurposing their identity fn to include the actual segment it's
referring to, instead of all of them

the `__next_prallelPaths` string doesn't seem to be used otherwise so I
think it's alright to rename


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- there's a warning that happens currently when compiling parallel
routes in dev because they all resolve to the same pathname
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Closes NEXT-831
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fix NEXT-831 ([link](https://linear.app/vercel/issue/NEXT-831))

packages/next/src/server/future/route-matchers/app-page-route-matcher.ts

@@ -3,8 +3,6 @@ import { AppPageRouteDefinition } from '../route-definitions/app-page-route-defi
 
 export class AppPageRouteMatcher extends RouteMatcher<AppPageRouteDefinition> {
   public get identity(): string {
-    return `${
-      this.definition.pathname
-    }?__nextParallelPaths=${this.definition.appPaths.join(',')}}`
+    return `${this.definition.pathname}?__nextPage=${this.definition.page}}`
   }
 }