[Dependency Scanning] Specify dependency inputs of Swift module dependencies on the command line

Do this by computing a transitive closure on the computed dependency graph, relying on the fact that it is a DAG.
The used algorithm is:
```
for each v ∈ V {
    T(v) = { v }
}
for v ∈ V in reverse topological order {
    for each (v, w) ∈ E {
         T(v) = T(v) ∪ T(w)
    }
}
```

lib/DependencyScan/ModuleDependencyCacheSerialization.cpp

@@ -656,19 +656,6 @@ struct llvm::DenseMapInfo<ModuleIdentifierArrayKind> {
   }
 };
 
-namespace std {
-template <>
-struct hash<ModuleDependencyID> {
-  using UnderlyingKindType = std::underlying_type<ModuleDependencyKind>::type;
-  std::size_t operator()(const ModuleDependencyID &id) const {
-    auto underlyingKindValue = static_cast<UnderlyingKindType>(id.second);
-
-    return (hash<string>()(id.first) ^
-            (hash<UnderlyingKindType>()(underlyingKindValue)));
-  }
-};
-} // namespace std
-
 namespace swift {
 
 class ModuleDependenciesCacheSerializer {