This commit adds functionality that allows the name resolution pass
to search for entities (traits/types/enums/structs) by name, in
order to show recommendations along with the errors.
For now, only E0405 and E0412 have suggestions attached, as per the
request in bug #21221, but it's likely other errors can also benefit
from the ability to generate suggestions.
This commit fixes all of the fallout of the previous commit which is an attempt
to refine privacy. There were a few unfortunate leaks which now must be plugged,
and the most horrible one is the current `shouldnt_be_public` module now inside
`std::rt`. I think that this either needs a slight reorganization of the
runtime, or otherwise it needs to just wait for the external users of these
modules to get replaced with their `rt` implementations.
Other fixes involve making things pub which should be pub, and otherwise
updating error messages that now reference privacy instead of referencing an
"unresolved name" (yay!).
