[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/Sema/ImportResolution.cpp

@@ -430,7 +430,7 @@ static void diagnoseNoSuchModule(ModuleDecl *importingModule,
     ctx.Diags.diagnose(importLoc, diagKind, modulePathStr);
   }
 
-  if (ctx.SearchPathOpts.SDKPath.empty() &&
+  if (ctx.SearchPathOpts.getSDKPath().empty() &&
       llvm::Triple(llvm::sys::getProcessTriple()).isMacOSX()) {
     ctx.Diags.diagnose(SourceLoc(), diag::sema_no_import_no_sdk);
     ctx.Diags.diagnose(SourceLoc(), diag::sema_no_import_no_sdk_xcrun);