[Serialization] Improve module loading performance

When looking for a Swift module on disk, we were scanning all module search paths if they contain the module we are searching for. In a setup where each module is contained in its own framework search path, this scaled quadratically with the number of modules being imported. E.g. a setup with 100 modules being imported form 100 module search paths could cause on the order of 10,000 checks of `FileSystem::exists`. While these checks are fairly fast (~10µs), they add up to ~100ms.

To improve this, perform a first scan of all module search paths and list the files they contain. From this, create a lookup map that maps filenames to the search paths they can be found in. E.g. for
```
searchPath1/
  Module1.framework

searchPath2/
  Module1.framework
  Module2.swiftmodule
```
we create the following lookup table
```
Module1.framework -> [searchPath1, searchPath2]
Module2.swiftmodule -> [searchPath2]
```

lib/Serialization/ModuleDependencyScanner.cpp

@@ -70,27 +70,23 @@ std::error_code ModuleDependencyScanner::findModuleFilesInDirectory(
   return dependencies.getError();
 }
 
-
-std::error_code PlaceholderSwiftModuleScanner::findModuleFilesInDirectory(
-    ImportPath::Element ModuleID, const SerializedModuleBaseName &BaseName,
-    SmallVectorImpl<char> *ModuleInterfacePath,
-    std::unique_ptr<llvm::MemoryBuffer> *ModuleBuffer,
-    std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer,
-    std::unique_ptr<llvm::MemoryBuffer> *ModuleSourceInfoBuffer,
-    bool skipBuildingInterface, bool IsFramework) {
-  StringRef moduleName = Ctx.getRealModuleName(ModuleID.Item).str();
+bool PlaceholderSwiftModuleScanner::findModule(
+    ImportPath::Element moduleID, SmallVectorImpl<char> *moduleInterfacePath,
+    std::unique_ptr<llvm::MemoryBuffer> *moduleBuffer,
+    std::unique_ptr<llvm::MemoryBuffer> *moduleDocBuffer,
+    std::unique_ptr<llvm::MemoryBuffer> *moduleSourceInfoBuffer,
+    bool skipBuildingInterface, bool &isFramework, bool &isSystemModule) {
+  StringRef moduleName = Ctx.getRealModuleName(moduleID.Item).str();
   auto it = PlaceholderDependencyModuleMap.find(moduleName);
-  // If no placeholder module stub path is given matches the name, return with an
-  // error code.
   if (it == PlaceholderDependencyModuleMap.end()) {
-    return std::make_error_code(std::errc::not_supported);
+    return false;
   }
   auto &moduleInfo = it->getValue();
   auto dependencies = ModuleDependencies::forPlaceholderSwiftModuleStub(
       moduleInfo.modulePath, moduleInfo.moduleDocPath,
       moduleInfo.moduleSourceInfoPath);
   this->dependencies = std::move(dependencies);
-  return std::error_code{};
+  return true;
 }
 
 static std::vector<std::string> getCompiledCandidates(ASTContext &ctx,