Add a new unstable flag -Ztreat-pub-as-pub-crate

[mu001999]

This PR adds a new unstable flag -Ztreat-pub-as-pub-crate as @Kobzol suggested.
When compiling binary crates with this flag, the seed worklist will only contain the entry fn and won't contain other reachable items. Then we can do the dead code analysis for pub items just like they are pub(crate).

Related zulip thread #general > pub/pub(crate) within a binary is a footgun.

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[Noratrieb]

What are the plans for this flag? Being made the default? As just a flag, it's essentially useless, because no one will know about it. So I'd be opposed to just adding it without any plan for making it useful for everyone.