[Serialization] Load indirect package dependencies from the current package

When loading a swiftmodule A, read its package information to tell if
the current client should load A's dependencies imports by a package
import. Only clients belonging to the same package as A should load
those dependencies, clients outside of the package likely don't have
access to those dependencies.

This is specific to swiftmodules as swiftinterfaces never display a
package-only import. Clients are unaware of package dependencies when
building against a swiftinterface.

rdar://106164813

lib/Serialization/ModuleFile.cpp

@@ -209,6 +209,10 @@ Status ModuleFile::associateWithFileContext(FileUnit *file, SourceLoc diagLoc,
       continue;
     }
 
+    if (dependency.isPackageOnly() &&
+        ctx.LangOpts.PackageName != this->getModulePackageName())
+      continue;
+
     ImportPath::Builder builder(ctx, dependency.Core.RawPath,
                                 /*separator=*/'\0');
     for (const auto &elem : builder) {