[red-knot] add support for typing_extensions.reveal_type (#13397)
Before `typing.reveal_type` existed, there was
`typing_extensions.reveal_type`. We should support both.
Also adds a test to verify that we can handle aliasing of `reveal_type`
to a different name.
Adds a bit of code to ensure that if we have a union of different
`reveal_type` functions (e.g. a union containing both
`typing_extensions.reveal_type` and `typing.reveal_type`) we still emit
the reveal-type diagnostic only once. This is probably unlikely in
practice, but it doesn't hurt to handle it smoothly. (It comes up now
because we don't support `version_info` checks yet, so
`typing_extensions.reveal_type` is actually that union.)
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
