This converts types in typing module to valid references when
`autodoc_unqualified_typehints` option enabled.

To support creating hyperlinks to container types naturally, py domain
should take fully-qualified typehints for them.  But nobody wants to
show "typing." module name on the signature. So this suppresses them
automatically.

… signature

To create hyperlinks to container types automatically, this prepends the
module names for the types under "typing" module.