PrebuiltModuleGen: dedupe module names before generating prebuilt modules.

We collect these Swift module names from the outputs of dependency scanning. However,
we've noticed dupes in the output, leading to build system reporting fatal errors. As a fix,
we dedupe the module names before generating prebuilt module cache.

rdar://158615938

Author: nkcsgexi

Sources/SwiftDriver/Jobs/PrebuiltModulesJob.swift

@@ -812,13 +812,13 @@ extension Driver {
       return (key, outputPaths)
     })
 
-    func collectSwiftModuleNames(_ ids: [ModuleDependencyId]) -> [String] {
-      return ids.compactMap { id in
+    func collectUniqueSwiftModuleNames(_ ids: [ModuleDependencyId]) -> [String] {
+      return Array(Set(ids.compactMap { id in
         if case .swift(let module) = id {
           return module
         }
         return nil
-      }
+      }))
     }
 
     func getSwiftDependencies(for module: String) -> [String] {
@@ -827,7 +827,7 @@ extension Driver {
       guard !dependencies.isEmpty else {
         return []
       }
-      return collectSwiftModuleNames(dependencies)
+      return collectUniqueSwiftModuleNames(dependencies)
     }
 
     func getOutputPaths(withName modules: [String], loadableFor arch: Triple.Arch) throws -> [TypedVirtualPath] {
@@ -870,7 +870,7 @@ extension Driver {
     // Keep track of modules we haven't handled.
     var unhandledModules = Set<String>(inputMap.keys)
     // Start from those modules explicitly imported into the file under scanning
-    var openModules = collectSwiftModuleNames(dependencyGraph.mainModule.allDependencies)
+    var openModules = collectUniqueSwiftModuleNames(dependencyGraph.mainModule.allDependencies)
     var idx = 0
     while idx != openModules.count {
       let module = openModules[idx]