[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)
    }
}
```

include/swift/Frontend/ModuleInterfaceLoader.h

@@ -129,7 +129,8 @@ class SearchPathOptions;
 class CompilerInvocation;
 
 /// A ModuleLoader that loads explicitly built Swift modules specified via
-/// -swift-module-file or modules found in
+/// -swift-module-file or modules found in a provided
+/// -explicit-swift-module-map-file JSON input.
 class ExplicitSwiftModuleLoader: public SerializedModuleLoaderBase {
   explicit ExplicitSwiftModuleLoader(ASTContext &ctx, DependencyTracker *tracker,
                                      ModuleLoadingMode loadMode,