[gatsby-plugin-vercel-builder] Fix nested SSR routes (#10751)

Fixes SSR / DSG pages that are nested deeper than the root path for Gatsby projects.

Also introduces unit tests for the logic related to determining which page name to use.

packages/gatsby-plugin-vercel-builder/templates/utils.ts

@@ -0,0 +1,29 @@
+import { parse } from 'url';
+import { basename, dirname } from 'path';
+
+export function getPageName(url: string, pathPrefix = '') {
+  let pathName = (parse(url).pathname || '/').slice(pathPrefix.length);
+  const isPageData = pathName.startsWith('/page-data/');
+  if (isPageData) {
+    // "/page-data/index/page-data.json" -> "/"
+    // "/page-data/using-ssr/page-data.json" -> "using-ssr"
+    // "/page-data/foo/bar/ssr/page-data.json" -> "foo/bar/ssr"
+    pathName = pathName.split('/').slice(2, -1).join('/');
+    if (pathName === 'index') {
+      pathName = '/';
+    }
+  } else {
+    // "/using-ssr" -> "using-ssr"
+    // "/using-ssr/" -> "using-ssr"
+    // "/using-ssr/index.html" -> "using-ssr"
+    // "/foo/bar/ssr" -> "foo/bar/ssr"
+    if (basename(pathName) === 'index.html') {
+      pathName = dirname(pathName);
+    }
+    if (pathName !== '/') {
+      // Remove leading and trailing "/"
+      pathName = pathName.replace(/(^\/|\/$)/g, '');
+    }
+  }
+  return { isPageData, pathName };
+}