So far, when deleting a library (either explicitely, or through e.g.
relocation), its dependencies would get a `nullptr` parent, and
therefore become 'directly used' libraries.
This commit adds a new util to rebuild the libraries hieararchy, and
calls it when a Library ID is deleted.
NOTE: While logic is somewhat similar to what liboverride resync does to
sort the libraries by indirect levels
(`lib_override_libraries_index_define`), there are some key differences
here, notably the fact that if a library has a valid `parent` pointer,
it is not replaced, even if a 'better' parent (less indirect library)
could be found.
1dca5af712