[NFC] Clarify semantics of getImportedModules.

The lack of clarity manifested as unexpected behavior when using
getImportedModules to create the module import graph. The new behavior
makes SPI-ness and Shadowing-ness behave similarly in terms of
filtering. We also check if a filter is well-formed to avoid
accidental empty import lists.

lib/AST/ImportCache.cpp

@@ -175,6 +175,7 @@ ImportSet &ImportCache::getImportSet(const DeclContext *dc) {
       ModuleDecl::ImportedModule{ImportPath::Access(), mod});
 
   if (file) {
+    // Should include both SPI & non-SPI.
     file->getImportedModules(imports,
                              {ModuleDecl::ImportFilterKind::Default,
                               ModuleDecl::ImportFilterKind::ImplementationOnly,
@@ -260,6 +261,7 @@ ImportCache::getAllAccessPathsNotShadowedBy(const ModuleDecl *mod,
       ModuleDecl::ImportedModule{ImportPath::Access(), currentMod});
 
   if (auto *file = dyn_cast<FileUnit>(dc)) {
+    // Should include both SPI & non-SPI
     file->getImportedModules(stack,
                              {ModuleDecl::ImportFilterKind::Default,
                               ModuleDecl::ImportFilterKind::ImplementationOnly,