prevent router markers from leaking (vercel#63606)

While looking into bugfixes related to router refreshing (the other PRs
in this stack), I noticed there were situations where the `Next-URL`
header would include `/children`, or where `page$` would be present in
LoaderTree for a segment.

This updates a few spots to prevent these markers from leaking into
places they shouldn't, and shouldn't have any behavioral changes.

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Closes NEXT-2902

packages/next/src/build/webpack/loaders/next-app-loader.ts

@@ -401,9 +401,14 @@ async function createTreeCodeFromPath(
         ? parallelSegment[0]
         : parallelSegment
 
+      // normalize the parallel segment key to remove any special markers that we inserted in the
+      // earlier logic (such as children$ and page$). These should never appear in the loader tree, and
+      // should instead be the corresponding segment keys (ie `__PAGE__`) or the `children` parallel route.
       parallelSegmentKey =
         parallelSegmentKey === PARALLEL_CHILDREN_SEGMENT
           ? 'children'
+          : parallelSegmentKey === PAGE_SEGMENT
+          ? PAGE_SEGMENT_KEY
           : parallelSegmentKey
 
       const normalizedParallelKey = normalizeParallelKey(parallelKey)

packages/next/src/client/components/router-reducer/compute-changed-path.ts

@@ -16,6 +16,10 @@ const removeLeadingSlash = (segment: string): string => {
 
 const segmentToPathname = (segment: Segment): string => {
   if (typeof segment === 'string') {
+    // 'children' is not a valid path -- it's technically a parallel route that corresponds with the current segment's page
+    // if we don't skip it, then the computed pathname might be something like `/children` which doesn't make sense.
+    if (segment === 'children') return ''
+
     return segment
   }
 
@@ -50,7 +54,7 @@ export function extractPathFromFlightRouterState(
 
   if (segment.startsWith(PAGE_SEGMENT_KEY)) return ''
 
-  const segments = [segment]
+  const segments = [segmentToPathname(segment)]
   const parallelRoutes = flightRouterState[1] ?? {}
 
   const childrenPath = parallelRoutes.children