No way to disable autobins

[ehuss]

A user on Discord noted that there is no way to completely disable automatic target discovery. Example:

src/lib.rs:

pub mod bin;

src/bin/mod.rs:

// empty

Currently this fails because it thinks there is a bin target called mod, regardless of whether autobins is specified.

I'd like to propose that autobins=false should disable autodiscovery, even if you don't specify any [[bin]] targets.

[alexcrichton]

This seems reasonable to me! I'm surprised that autobins = false doesn't already work, and I'm also somewhat surprised that it's connected to the contents of src/lib.rs!