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BPath traversing: allow skipping weak library references

Add flag to `BKE_bpath_traverse_id()` and friends to skip weak
references (see below). This makes a distinction between "this blend
file depends on that file" and "this blend file references that file,
but doesn't directly use its data". This distinction is for the Asset
Bundle install operator, which refuses to copy the blend file when it's
not self-contained.

Weak references are those that are not directly used by the blend file,
but are still present to allow path rewriting. For example, when an
Asset is loaded its originating blend file is saved in
`ID::library_weak_reference`; this reference is purely for deduplication
purposes, and not for actually loading any data.

Reviewed by: mont29, brecht

Differential Revision: https://developer.blender.org/D13412