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/startup/bl_ui/space_graph.py

@@ -384,7 +384,7 @@ class GRAPH_MT_key_density(Menu):
     bl_label = "Density"
 
     def draw(self, _context):
-        from bl_ui_utils.layout import operator_context
+        from _bl_ui_utils.layout import operator_context
         layout = self.layout
         layout.operator("graph.decimate", text="Decimate (Ratio)").mode = 'RATIO'
         # Using the modal operation doesn't make sense for this variant