Core: Add packed linked data-blocks

This adds support for packed linked data. This is a key part of an improved
asset workflow in Blender.

Packed IDs remain considered as linked data (i.e. they cannot be edited),
but they are stored in the current blendfile. This means that they:
* Are not lost in case the library data becomes unavailable.
* Are not changed in case the library data is updated.

These packed IDs are de-duplicated across blend-files, so e.g. if a shot
file and several of its dependencies all use the same util geometry node,
there will be a single copy of that geometry node in the shot file.

In case there are several versions of a same ID (e.g. linked at different
moments from a same library, which has been modified in-between), there
will be several packed IDs.

Name collisions are averted by storing these packed IDs into a new type of
'archive' libraries (and their namespaces). These libraries:
* Only contain packed IDs.
* Are owned and managed by their 'real' library data-block, called an
  'archive parent'.

For more in-depth, technical design: #132167
UI/UX design: #140870

Co-authored-by: Bastien Montagne <bastien@blender.org>
Pull Request: https://projects.blender.org/blender/blender/pulls/133801

source/blender/python/intern/bpy_library_load.cc

@@ -223,6 +223,9 @@ PyDoc_STRVAR(
     "   :type filepath: str | bytes\n"
     "   :arg link: When False reference to the original file is lost.\n"
     "   :type link: bool\n"
+    "   :arg pack: If True, and ``link`` is also True, pack linked data-blocks into the current "
+    "blend-file.\n"
+    "   :type pack: bool\n"
     "   :arg relative: When True the path is stored relative to the open blend file.\n"
     "   :type relative: bool\n"
     "   :arg set_fake: If True, set fake user on appended IDs.\n"
@@ -260,6 +263,7 @@ static PyObject *bpy_lib_load(BPy_PropertyRNA *self, PyObject *args, PyObject *k
    */
   struct {
     BoolFlagPair is_link = {false, FILE_LINK};
+    BoolFlagPair is_pack = {false, BLO_LIBLINK_PACK};
     BoolFlagPair is_relative = {false, FILE_RELPATH};
     BoolFlagPair set_fake = {false, BLO_LIBLINK_APPEND_SET_FAKEUSER};
     BoolFlagPair recursive = {false, BLO_LIBLINK_APPEND_RECURSIVE};
@@ -279,6 +283,7 @@ static PyObject *bpy_lib_load(BPy_PropertyRNA *self, PyObject *args, PyObject *k
   static const char *_keywords[] = {
       "filepath",
       "link",
+      "pack",
       "relative",
       "set_fake",
       "recursive",
@@ -296,6 +301,7 @@ static PyObject *bpy_lib_load(BPy_PropertyRNA *self, PyObject *args, PyObject *k
       /* Optional keyword only arguments. */
       "|$"
       "O&" /* `link` */
+      "O&" /* `pack` */
       "O&" /* `relative` */
       "O&" /* `recursive` */
       "O&" /* `set_fake` */
@@ -317,6 +323,8 @@ static PyObject *bpy_lib_load(BPy_PropertyRNA *self, PyObject *args, PyObject *k
                                         PyC_ParseBool,
                                         &flag_vars.is_link,
                                         PyC_ParseBool,
+                                        &flag_vars.is_pack,
+                                        PyC_ParseBool,
                                         &flag_vars.is_relative,
                                         PyC_ParseBool,
                                         &flag_vars.recursive,
@@ -389,10 +397,18 @@ static PyObject *bpy_lib_load(BPy_PropertyRNA *self, PyObject *args, PyObject *k
       PyErr_SetString(PyExc_ValueError, "`link` is False but `create_liboverrides` is True");
       return nullptr;
     }
+    if (flag_vars.is_pack.value) {
+      PyErr_SetString(PyExc_ValueError, "`pack` must be False if `link` is False");
+      return nullptr;
+    }
   }
 
   if (create_liboverrides) {
     /* Library overrides. */
+    if (flag_vars.is_pack.value) {
+      PyErr_SetString(PyExc_ValueError, "`create_liboverrides` must be False if `pack` is True");
+      return nullptr;
+    }
   }
   else {
     /* Library overrides (disabled). */
@@ -639,6 +655,7 @@ static bool bpy_lib_exit_lapp_context_items_cb(BlendfileLinkAppendContext *lapp_
 static PyObject *bpy_lib_exit(BPy_Library *self, PyObject * /*args*/)
 {
   Main *bmain = self->bmain;
+  const bool do_pack = ((self->flag & BLO_LIBLINK_PACK) != 0);
   const bool do_append = ((self->flag & FILE_LINK) == 0);
   const bool create_liboverrides = self->create_liboverrides;
   /* Code in #bpy_lib_load should have raised exception in case of incompatible parameter values.
@@ -709,7 +726,10 @@ static PyObject *bpy_lib_exit(BPy_Library *self, PyObject * /*args*/)
   BKE_blendfile_link_append_context_init_done(lapp_context);
 
   BKE_blendfile_link(lapp_context, nullptr);
-  if (do_append) {
+  if (do_pack) {
+    BKE_blendfile_link_pack(lapp_context, nullptr);
+  }
+  else if (do_append) {
     BKE_blendfile_append(lapp_context, nullptr);
   }
   else if (create_liboverrides) {