Introduce a package interface.

It has an extension .package.swiftinterface and contains package decls
as well as SPIs and public/inlinable decls. When a module is loaded
from interface, it now looks up the package-name in the interface
and checks if the importer is in the same package. If so, it uses
that package interface found to load the module. If not, uses the existing
logic to load modules.

Resolves rdar://104617854

lib/Frontend/CompilerInvocation.cpp

@@ -417,7 +417,7 @@ static void ParseModuleInterfaceArgs(ModuleInterfaceOptions &Opts,
   if (const Arg *A = Args.getLastArg(OPT_library_level)) {
     StringRef contents = A->getValue();
     if (contents == "spi") {
-      Opts.PrintPrivateInterfaceContent = true;
+      Opts.InterfaceContentMode = PrintInterfaceContentMode::Private;
     }
   }
   for (auto val: Args.getAllArgValues(OPT_skip_import_in_public_interface)) {