Previously, rustdoc had 3 fallbacks it used:
1. `resolve_macro_path`
2. `all_macros`
3. `resolve_str_path_error`
Ideally, it would only use `resolve_str_path_error`, to be consistent with other namespaces.
Unfortunately, that doesn't consider macros that aren't defined at module scope;
consider for instance
```rust
{
struct S;
macro_rules! mac { () => {} }
// `mac`'s scope starts here
/// `mac` <- `resolve_str_path_error` won't see this
struct Z;
//`mac`'s scope ends here
}
```
This changes it to only use `all_macros` and `resolve_str_path_error`, and gives
`resolve_str_path_error` precedence over `all_macros` in case there are two macros with the same
name in the same module.
This also adds a failing test case which will catch trying to remove `all_macros`.
