LibLink/Append: Some cleanup and addition to comments.

Rewrite ID Paste code to use new BKE_blendfile_link_append module.

No behavioral changes are expected here.

Part of T91414: Unify link/append between WM operators and BPY context
manager API, and cleanup usages of `BKE_library_make_local`.

LibLink/Append: Add a utils to link/append all ID of given types from a given library.

This will be used by the copy/paste code.

Part of T91414: Unify link/append between WM operators and BPY context
manager API, and cleanup usages of `BKE_library_make_local`.

LibLink/Append: tweak instantiation of collections in viewlayers.

When appended collections are instantiated in viewlayers, do not add to
viewlayers collections children of other instantiated collections.

This default behavior is required for proper copy/paste behavior. If
previous behavior needs to be brought back for regular append operation,
this can be decided and done, but think this default behavior also makes
sense in normal append case.

part of T91414: Unify link/append between WM operators and BPY context
manager API, and cleanup usages of `BKE_library_make_local`.

Fix T93353: Reload Library Override file loses Constraints, take II.

When adding `INSERT` operations over RNACollection items, rna diffing
code did not properly report the properties as not being equals.

This in turn triggered the 'purge unused exiting override properties'
mechanism, thus deleting the exitsting (valid) insert override property
operation.

NOTE: This should also be backported to 2.93, and probably 2.83.

Reviewed By: sybren, jbakker

Maniphest Tasks: T93353

Differential Revision: https://developer.blender.org/D13426

LibLink/Append: Port `bpy.data.libraries.load` to new `BKE_blendfile_link_append` module.

Note that this fully replaces the 'PyCapsule' storage of linked/appended items
in the python API code by the generic storage of items in the
`BlendfileLinkAppendContext` data.

Maniphest Tasks: T91414

Differential Revision: https://developer.blender.org/D13331

LibLink/Append: tweak asserts in main BKE link/append functions.

Now that those functions are much widely used, just return early in case
there is nothing to link/append, instead of asserting over it.

Refactor BKE_bpath module.

The main goal of this refactor is to make BPath module use `IDTypeInfo`,
and move each ID-specific part of the `foreach_path` looper into their
own IDTypeInfo struct, using a new `foreach_path` callback.

Additionally, following improvements/cleanups are included:
* Attempt to get better, more consistent namings.
** In particular, move from `path_visitor` to more standard `foreach_path`.
* Update and extend documentation.
** API doc was moved to header, according to recent discussions on this
topic.
* Remove `BKE_bpath_relocate_visitor` from API, this is specific
callback that belongs in `lib_id.c` user code.

NOTE: This commit is expected to be 100% non-behavioral-change. This
implies that several potential further changes were only noted as
comments (like using a more generic solution for
`lib_id_library_local_paths`, addressing inconsistencies like path of
packed libraries always being skipped, regardless of the
`BKE_BPATH_FOREACH_PATH_SKIP_PACKED` `eBPathForeachFlag` flag value,
etc.).

NOTE: basic unittests were added to master already in
rBdcc500e5a265093bc9cc.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D13381

Attempt to fix Windows new bpath tests failing, take IV.

Follow up to rBdcc500e5a265093bc9cc, rB92daff6ac2adb5bb,
rB61bd5882a20c6f3 and rB08264aaf82da8.

Sorry for the noise, this time should be good.

Attempt to fix Windows new bpath tests failing, take III.

Follow up to rBdcc500e5a265093bc9cc, rB92daff6ac2adb5bb and rB61bd5882a20c6f3.