Support both --bin and --lib together in cargo new

[kornelski]

Fixes #5431

[rust-highfive]

r? @matklad

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[matklad]

We've discussed this at the Cargo meeting, and the conclusion was that, while today src/lib.rs + src/main.rs is the default pattern for applications, we are actually not sure that this is the proper way to structure code, and not just a tradition and a historical accident.

So, we are not ready to promote this pattern even more, until we decide that this is indeed a good pattern to promote. The discussion about that happens at rust-lang/api-guidelines#167.

Let's keep this PR open, in case that that discussion ends quickly and in favor of the current pattern!

[alexcrichton]

I'm gonna close this in favor of rust-lang/api-guidelines#167 for now as it's been awhile, but we can of course reopen if the discussion comes to a close!