Rollup merge of #122665 - ehuss:pub-priv-tests, r=davidtwco

Add some tests for public-private dependencies.

This adds some tests to show more scenarios for the `exported_private_dependencies` lint. Several of these illustrate that the lint is not working as expected, and I have annotated those places with `FIXME`.

Note also that this includes some diamond dependency structures which compiletest doesn't exactly support. However, I don't think it should be a problem, it just results in the common dependency being built twice.

Author: matthiaskrgr