PyAPI: make internal modules explicitly "private"

Rename modules in `./scripts/modules/` to use an underscore prefix to
make it clear they aren't intended to be part of public API's. This
also means there is no implication that these modules should be stable,
allowing us to change them based on Blender's internal usage.

The following modules have been marked as private:

- `animsys_refactor`
- `bl_console_utils`
- `bl_i18n_utils`
- `bl_previews_utils`
- `bl_rna_utils`
- `bl_text_utils`
- `bl_ui_utils`
- `bpy_restrict_state`
- `console_python`
- `console_shell`
- `graphviz_export`
- `keyingsets_utils`
- `rna_info`
- `rna_manual_reference`
- `rna_xml`

Note that we could further re-arrange these modules
(under `_bpy_internal` in some cases), this change is mainly to mark
them as private, further changes can be handed on a case-by-case basis.

Ref !147773

scripts/modules/bpy/utils/__init__.py

@@ -321,7 +321,7 @@ def load_scripts(*, reload_scripts=False, refresh_scripts=False, extensions=True
         # Without this, add-on register functions accessing key-map properties can crash, see: #111702.
         _bpy.context.window_manager.keyconfigs.update(keep_properties=True)
 
-    from bpy_restrict_state import RestrictBlend
+    from _bpy_restrict_state import RestrictBlend
 
     with RestrictBlend():
         for base_path in script_paths(use_user=use_user):
@@ -367,7 +367,7 @@ def _on_exit():
 
     # Call `unregister` function on internal startup module.
     # Must only be used as part of Blender 'exit' process.
-    from bpy_restrict_state import RestrictBlend
+    from _bpy_restrict_state import RestrictBlend
     with RestrictBlend():
         for mod_name in reversed(_registered_module_names):
             if (mod := _sys.modules.get(mod_name)) is None:
@@ -1231,7 +1231,7 @@ def _blender_default_map():
     # NOTE(@ideasman42): Avoid importing this as there is no need to keep the lookup table in memory.
     # As this runs when the user accesses the "Online Manual", the overhead loading the file is acceptable.
     # In my tests it's under 1/100th of a second loading from a `pyc`.
-    ref_mod = execfile(_os.path.join(_script_base_dir, "modules", "rna_manual_reference.py"))
+    ref_mod = execfile(_os.path.join(_script_base_dir, "modules", "_rna_manual_reference.py"))
     return (ref_mod.url_manual_prefix, ref_mod.url_manual_mapping)