3707: Add ItemScope::visibility_of r=matklad a=edwin0cheng
~This PR implements `HasVisibility` for various constructs and change `Definition::search_scope` to use `Visibility` directly instead of depends on ad-hoc string parsing.~
This PR added `visibility_of` in `ItemScope` and `Module` and use it directly directly instead of depends on ad-hoc string parsing.
And also add a FIXME to indicate that there is a bug which do not search child-submodules in other files recursively in `Definition::search_scope`.
I will submit another PR to fix that bug after this is merged.
cc @flodiebold
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
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