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

src/test/compile-fail-fulldeps/macro-crate-doesnt-resolve.rs

@@ -14,6 +14,5 @@
 extern crate macro_crate_test;
 
 fn main() {
-    macro_crate_test::foo();
-    //~^ ERROR failed to resolve. Use of undeclared type or module `macro_crate_test`
+    macro_crate_test::foo(); //~ ERROR unresolved name
 }

src/test/compile-fail-fulldeps/no-link-unknown-crate.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#[macro_use] #[no_link]
+#[no_link]
 extern crate doesnt_exist; //~ ERROR can't find crate
 
 fn main() {}

src/test/compile-fail/no-link.rs

@@ -13,6 +13,6 @@ extern crate libc;
 
 fn main() {
     unsafe {
-        libc::abs(0);  //~ ERROR Use of undeclared type or module `libc`
+        libc::abs(0);  //~ ERROR unresolved name
     }
 }