Make dependency_formats an FxIndexMap rather than a list of tuples

It is treated as a map already. This is using FxIndexMap rather than
UnordMap because the latter doesn't provide an api to pick a single
value iff all values are equal, which each_linked_rlib depends on.

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -1749,7 +1749,7 @@ fn for_each_exported_symbols_include_dep<'tcx>(
     }
 
     let formats = tcx.dependency_formats(());
-    let deps = formats.iter().find_map(|(t, list)| (*t == crate_type).then_some(list)).unwrap();
+    let deps = &formats[&crate_type];
 
     for (index, dep_format) in deps.iter().enumerate() {
         let cnum = CrateNum::new(index + 1);