Lint if explicit `package.readme` could be inferred

### What it does

Cargo can infer README to where it doesn't need to be set

### Advantage

Reduce boilerplate

Avoid #14020

### Drawbacks

People have to hope they got the inferring logic right

### Example

```toml
[package]
readme = "README.md"
```

Could be written as:

```toml
[package]
```
