add indirection for user-scope searches

When viewing documenation for a user-scoped package, the search box in
the top left was originally meant to find the documentation entry
point using a `PLT_Root...` cookie, which migrated to mean
`PLT_Root...` local storage. The local-storage approach stopped
working years ago, as browsers made different file count as different
domains.

This change enables a layer like `racket-index` to define cause
`user_doc_root` to be defined, which it can do as part of the
`local-redirect` layer that is alreayd in place. And that makes the
top-left search box work again.

scribble-lib/scribble/scribble-common.js

@@ -70,14 +70,24 @@ function SetPLTRoot(ver, relative) {
 
 // adding index.html works because of the above
 function GotoPLTRoot(ver, relative) {
-  var u = GetCookie("PLT_Root."+ver, null);
+  var u = GetRootPath(ver);
   if (u == null) return true; // no cookie: use plain up link
   // the relative path is optional, default goes to the toplevel start page
   if (!relative) relative = "index.html";
   location = u + relative;
   return false;
 }
 
+function GetRootPath(ver) {
+    var u = GetCookie("PLT_Root."+ver, null);
+    if (u != null)
+        return u;
+    // use root specified by local-redirect wrapper, if present
+    if (typeof user_doc_root != "undefined")
+        return user_doc_root;
+    return null;
+}
+
 // Utilities ------------------------------------------------------------------
 
 var normalize_rxs = [/\/\/+/g, /\/\.(\/|$)/, /\/[^\/]*\/\.\.(\/|$)/];
@@ -97,7 +107,7 @@ function NormalizePath(path) {
 function DoSearchKey(event, field, ver, top_path) {
   var val = field.value;
   if (event && event.key === 'Enter') {
-    var u = GetCookie("PLT_Root."+ver, null);
+    var u = GetRootPath(ver);
     if (u == null) u = top_path; // default: go to the top path
     u += "search/index.html?q=" + encodeURIComponent(val);
     u = MergePageArgsIntoUrl(u);