[Serialization] Differentiate module loading behavior for non-public imports

Differentiate `internal` and `fileprivate` imports from
implementation-only imports at the module-wide level to offer a
different module loading strategy. The main difference is for non-public
imports from a module with testing enabled to be loaded by transitive
clients.

Ideally, we would only load transitive non-public dependencies on
testable imports of the middle module. The current module loading logic
doesn't allow for this behavior easily as a module may be first loaded
for a normal import and extra dependencies would have to be loaded on
later imports. We may want to refactor the module loading logic to allow
this if needed.

rdar://106514965

lib/Serialization/ModuleFile.cpp

@@ -468,6 +468,10 @@ void ModuleFile::getImportedModules(SmallVectorImpl<ImportedModule> &results,
         continue;
       }
 
+    } else if (dep.isInternalOrBelow()) {
+      if (!filter.contains(ModuleDecl::ImportFilterKind::InternalOrBelow))
+        continue;
+
     } else if (dep.isPackageOnly()) {
       if (!filter.contains(ModuleDecl::ImportFilterKind::PackageOnly))
         continue;