Auto merge of #34032 - jseyfried:load_macros_in_expansion, r=nrc

Auto merge of #34032 - jseyfried:load_macros_in_expansion, r=nrc

Support `#[macro_use]` on macro-expanded crates

This PR loads macros from `#[macro_use]` crates during expansion so that
 - macro-expanded `#[macro_use]` crates work (fixes #33936, fixes #28071), and
 - macros imported from crates have the same scope as macros imported from modules.

This is a [breaking-change]. For example, this will break:
```rust
macro_rules! m {
    () => { #[macro_use(foo)] extern crate core; } //~ ERROR imported macro not found
}
m!();
```
Also, this will break:
```rust
macro_rules! try { () => {} }
// #[macro_use] mod bar { macro_rules! try { ... } } //< ... just like this would ...
fn main() { try!(); } //< ... making this an error
```

r? @nrc