Previously, if there were a module in scope with the same name as the
primitive, that would take precedence. Coupled with
https://github.com/rust-lang/rust/issues/58699, this made it impossible
to link to the primitive when that module was in scope.

This approach could be extended so that `struct@foo` would no longer resolve
to any type, etc. However, it could not be used for glob imports:

```rust
pub mod foo {
  pub struct Bar;
}

pub enum Bar {}
use foo::*;

// This is expected to link to `inner::Bar`, but instead it will link to the enum.
/// Link to [struct@Bar]
pub struct MyDocs;
```

The reason for this is that this change does not affect the resolution
algorithm of rustc_resolve at all. The only reason we could special-case
primitives is because we have a list of all possible primitives ahead of time.