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/_bl_i18n_utils/utils_languages_menu.py

@@ -0,0 +1,68 @@
+# SPDX-FileCopyrightText: 2013-2023 Blender Authors
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# Update "languages" text file used by Blender at runtime to build translations menu.
+
+
+OK = 0
+MISSING = 1
+TOOLOW = 2
+SKIPPED = 3
+FLAG_MESSAGES = {
+    OK: "",
+    MISSING: "No translation yet.",
+    TOOLOW: "Not complete enough to be included.",
+    SKIPPED: "Skipped (see IMPORT_LANGUAGES_SKIP in settings.py).",
+}
+
+
+def gen_menu_file(stats, settings):
+    # Generate languages file content used by Blender's i18n system.
+    # First, match all entries in LANGUAGES to a `lang` in stats, if possible!
+    # Returns a iterable of text lines.
+    tmp = []
+    for uid_num, label, uid in settings.LANGUAGES:
+        if uid in stats:
+            if uid in settings.IMPORT_LANGUAGES_SKIP:
+                tmp.append((stats[uid], uid_num, label, uid, SKIPPED))
+            else:
+                tmp.append((stats[uid], uid_num, label, uid, OK))
+        else:
+            tmp.append((0.0, uid_num, label, uid, MISSING))
+    stats = tmp
+    stats = sorted(stats, key=lambda it: it[0], reverse=True)
+    langs = []
+    highest_uid = 0
+    for lvl, uid_num, label, uid, flag in stats:
+        if lvl < settings.IMPORT_MIN_LEVEL and flag == OK:
+            flag = TOOLOW
+        if uid_num == 0:
+            # Insert default language (Automatic) at index 0, after sorting.
+            default_lang = (uid_num, label, uid, flag, lvl)
+            continue
+        langs.append((uid_num, label, uid, flag, lvl))
+        if abs(uid_num) > highest_uid:
+            highest_uid = abs(uid_num)
+    # Sort languages by name.
+    langs.sort(key=lambda it: it[1])
+    langs.insert(0, default_lang)
+    data_lines = [
+        "# File used by Blender to know which languages (translations) are available, ",
+        "# and to generate translation menu.",
+        "#",
+        "# File format:",
+        "# ID:MENULABEL:ISOCODE",
+        "# ID must be unique, except for 0 value (marks categories for menu).",
+        "# Line starting with a # are comments!",
+        "#",
+        "# Automatically generated by _bl_i18n_utils/utils_languages_menu.py script.",
+        "# Highest ID currently in use: {}".format(highest_uid),
+    ]
+    for uid_num, label, uid, flag, lvl in langs:
+        if flag == OK:
+            data_lines.append("{}:{}:{}:{}%".format(uid_num, label, uid, round(lvl * 100)))
+        else:
+            # Non-existing, commented entry!
+            data_lines.append("# {} #{}:{}:{}:{}%".format(FLAG_MESSAGES[flag], uid_num, label, uid, round(lvl * 100)))
+    return data_lines