Autocomplete/autofill doesn't update when Autoload scene root is given new script

### Tested versions

Tested in:
v4.8.dev3.official [51105ccbe]
v4.7.stable.official [5b4e0cb0f]

### System information

Windows 10

### Issue description

The mechanism responsible for updating the autocomplete information of an autoload scene will only be triggered by changes made to an external script file that was attached when the autoload was registered.

Some notes:
- Built-in scripts do not trigger updates at all.
- If the Autoload's script is removed and replaced with a new one, the original script will continue to update the autocomplete as though it was still attached.
- Restarting the editor will temporarily fix the issue.

### Steps to reproduce

1. Register a scene with a root node that has no attached script.
2. Attach a script.
3. In another script, observe the discrepancy in the autocompletion.

### Minimal reproduction project (MRP)

N/A

Start by reproducing the issue in the Godot editor with an autoload scene whose root script is attached after registration, then trace the mechanism that updates autoload autocomplete information. Check the built-in-script and script-replacement cases as well. Done means autocomplete updates for newly attached or replacement scripts, including built-in scripts, without stale updates from the old script.

Written by the indexing model from the issue text.