Auto merge of #37247 - jseyfried:future_proof_no_link, r=nrc

macros: Future proof `#[no_link]`

This PR future proofs `#[no_link]` for macro modularization (cc #35896).

First, we resolve all `#[no_link] extern crate`s. `#[no_link]` crates without `#[macro_use]` or `#[macro_reexport]` are not resolved today, this is a [breaking-change]. For example,
```rust
```
Any breakage can be fixed by simply removing the `#[no_link] extern crate`.

Second, `#[no_link] extern crate`s will define an empty module in type namespace to eventually allow importing the crate's macros with `use`. This is a [breaking-change], for example:
```rust
mod syntax {} //< This becomes a duplicate error.
```

r? @nrc

1 files changed, 1 insertions, 1 deletions

diff --git a/src/librustc_resolve/build_reduced_graph.rs b/src/librustc_resolve/build_reduced_graph.rs
index f19afd67faa..9ed7be5af4e 100644
--- a/src/librustc_resolve/build_reduced_graph.rs
+++ b/src/librustc_resolve/build_reduced_graph.rs
@@ -267,7 +267,7 @@ impl<'b> Resolver<'b> {
                     }
 
                     self.populate_module_if_necessary(module);
-                } else if custom_derive_crate {
+                } else {
                     // Define an empty module
                     let def = Def::Mod(self.definitions.local_def_id(item.id));
                     let module = ModuleS::new(Some(parent), ModuleKind::Def(def, name));