Limit `exported-private-dependencies` lints to libraries

[epage]

Problem

When developing bins or tests, the use of a dependency in a pub item doesn't actually make it a public dependency.

Proposed Solution

Restrict which targets use -extern priv:

Notes

No response

[linyihai]

@rustbot claim

[epage]

FYI likely the first step is adding tests like https://github.com/rust-lang/cargo/blob/master/tests/testsuite/pub_priv.rs#L7 to cover tests, bins, build scripts, benches, examples, etc. If we need updating, the code is at https://github.com/rust-lang/cargo/blob/master/src/cargo/core/compiler/mod.rs#L1430

[epage]

Closed via #13135