Extract autodoc importing to module-level functions (sphinx-doc#13773)

The import functions are now simple functions that take a module and
path to import and return an object representing the successful
import. Previously, substantial amounts of state were implicit, or
set directly onto the `Documenter` subclass. This should help with
testing in the future, as autodoc importing no longer requires any
of the core Sphinx classes. Finally, this means we can simplify the
import functions by splitting apart different object types, removing
single purpose logic from the main ``_import_obect()`` function.

Author: AA-Turner