Rather than them being set and stored globally, make them members of
`GIRepository`. This helps us move away from the concept of a global
singleton `GIRepository`.
This is slightly complicated by the fact that the library paths are
needed within the module loading code in `GITypelib`, but at that point
the `GITypelib` doesn’t have access to its parent `GIRepository` to call
`gi_repository_get_library_path()`, so we have to cache them in
`typelib->library_paths`.
It also means that it’s no longer possible to retrieve the ‘unset’ paths
from the globals, so the test for that is removed from
`repository-search-paths.c`.
This commit makes some API breaks, but that’s OK because libgirepository
has not been in a stable release yet.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: #3155
