Auto merge of #5335 - dwijnand:target-autodiscovery, r=alexcrichton

Auto merge of #5335 - dwijnand:target-autodiscovery, r=alexcrichton

Introduce autoXXX keys for target auto-discovery

In Rust 2015 absence of the configuration makes it default to not
include auto-discovered targets (i.e false), with a warnings message.

In Rust 2018 absence makes it default to include auto-discovered
targets (i.e true).

Fixes #5330

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_original_

Fixes #5330

submitted for early review. feedback very welcome, I'm happy to iterate (and learn).

in particular I require assistance with the borrowing of `warnings` in `clean_benches`.