Some registries may contain multiple modules (e.g., Python contains all
the standard library modules, or Matplotlib contains modules in the
`mpl_toolkits` namespace), and the previous lookup by the intersphinx
mapping _only_ could cause these to be missed. There was already some
special casing for Python, but this is now extended to all inventories,
fixing the case for Matplotlib as well.
Fixes matplotlib/matplotlib#28250