make `make_instance` available as `type_of_module_type`
raise DisplayTypes if `Type_not_found` occurs in display mode with a known module (see #3288)
add raise version of `string_list_of_expr_path`
use DisplayFields instead of DisplayTypes for submodule completion (see #3288)
deal with empty module types and sub-type completion
tests for #3288
Merge pull request #3304 from nadako/issue_3288

Support module subtypes in completion (closes #3288)