fix: properly support index routes with a path in useResolvedPath

[brophdawg11]

While not a super common pattern, this was causing Form and useSubmit to use incorrect default actions when used in an index route with a path (more commonly generated from remix conventional routes):

<Route path="/">
  <Route path="foo" index={true} element={<Comp />} />
</Route>

A call to useFormAction inside Comp would incorrectly trim the foo route in getPathContributingMatches because it was an index route, and then it would generate a form action of /?index instead of /foo?index

[changeset-bot]

🦋 Changeset detected

Latest commit: ce47c0a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
react-router-dom	Patch
@remix-run/router	Patch
react-router	Patch
react-router-dom-v5-compat	Patch
react-router-native	Patch

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[brophdawg11]

Removed the duped function and exported it as a private function from @remix-run/router

[brophdawg11]

Don't look for match.route.index at all - just look to see if the pathnameBase changed

[brophdawg11]

2 issues fixed here:

• Looking at route.index doesn't work since index routes can have paths
• Using pathnameBase doesn't work for nested splat routes since they don't add to pathnameBase, so instead we just look for the route do define a non-empty path value (also this means this becomes a function that we can use directly in remix since remix doesn't expose pathnameBase on matches)