Move access-path filtering into ModuleNameLookup (out of lookupValue) (#27097)

Removes duplicated logic from the implementations of
FileUnit::lookupValue, and simplifies the interface to
ModuleDecl::lookupValue, where everyone was passing an empty
(non-filtering) access path anyway /except/ during actual lookup from
source code. No functionality change.

lib/AST/ModuleNameLookup.cpp

@@ -80,7 +80,11 @@ private:
 
   void doLocalLookup(ModuleDecl *module, ModuleDecl::AccessPathTy path,
                      SmallVectorImpl<ValueDecl *> &localDecls) {
-    module->lookupValue(path, name, lookupKind, localDecls);
+    // If this import is specific to some named decl ("import Swift.Int")
+    // then filter out any lookups that don't match.
+    if (!ModuleDecl::matchesAccessPath(path, name))
+      return;
+    module->lookupValue(name, lookupKind, localDecls);
   }
 };