Get or create import function

[guybedford]

Currently add_import_function on Module will allow adding duplicate imports, which is usually undesired.

A new get_or_create_import_function primitive would be very useful to enable idempotent semantics and avoiding of duplicate imports. Alternatively the existing add_import_function could possible be extended, but we are focusing on full backwards compat on the API for the most part, preferring to deprecate APIs over changing them.

[guybedford]

Similiar functionality could also be useful for adding types and other types that have some level of equality. Of course type equality should also be performed for get_or_create_import_function, throwing an error if the types are different for the same import.