Don't show non-public names from buitlins stub in completion (PY-22828)

The Typeshed stub from buitlins contains several non-public names:

* Imports from other modules
* Protected names (_foo)

python/src/com/jetbrains/python/psi/impl/references/PyReferenceImpl.java

@@ -637,7 +637,19 @@ public class PyReferenceImpl implements PsiReferenceEx, PsiPolyVariantReference
 
     // include our own names
     final int underscores = PyUtil.getInitialUnderscores(element.getName());
-    final CompletionVariantsProcessor processor = new CompletionVariantsProcessor(element);
+    final PyBuiltinCache builtinCache = PyBuiltinCache.getInstance(element);
+    final CompletionVariantsProcessor processor = new CompletionVariantsProcessor(element, e -> {
+      if (builtinCache.isBuiltin(e)) {
+        final String name = e instanceof PyElement ? ((PyElement)e).getName() : null;
+        if (e instanceof PyImportElement) {
+          return false;
+        }
+        if (name != null && PyUtil.getInitialUnderscores(name) == 1) {
+          return false;
+        }
+      }
+      return true;
+    }, null);
     final ScopeOwner owner = realContext instanceof ScopeOwner ? (ScopeOwner)realContext : ScopeUtil.getScopeOwner(realContext);
     if (owner != null) {
       PyResolveUtil.scopeCrawlUp(processor, owner, null, null);
@@ -648,7 +660,7 @@ public class PyReferenceImpl implements PsiReferenceEx, PsiPolyVariantReference
     KeywordArgumentCompletionUtil.collectFunctionArgNames(element, ret, TypeEvalContext.codeCompletion(element.getProject(), element.getContainingFile()));
 
     // include builtin names
-    final PyFile builtinsFile = PyBuiltinCache.getInstance(element).getBuiltinsFile();
+    final PyFile builtinsFile = builtinCache.getBuiltinsFile();
     if (builtinsFile != null) {
       PyResolveUtil.scopeCrawlUp(processor, builtinsFile, null, null);
     }

python/src/com/jetbrains/python/psi/resolve/PythonBuiltinReferenceResolveProvider.java

@@ -59,7 +59,7 @@ public class PythonBuiltinReferenceResolveProvider implements PyReferenceResolve
 
     // ...as a builtin symbol
     final PyFile builtinsFile = builtinCache.getBuiltinsFile();
-    if (builtinsFile != null && !PyUtil.isClassPrivateName(referencedName)) {
+    if (builtinsFile != null && !PyUtil.isClassPrivateName(referencedName) && PyUtil.getInitialUnderscores(referencedName) != 1) {
       for (RatedResolveResult resolveResult : builtinsFile.multiResolveName(referencedName)) {
         result.add(new ImportedResolveResult(resolveResult.getElement(), resolveResult.getRate(), null));
       }

python/testData/inspections/PyUnresolvedReferencesInspection/noProtectedBuiltinNames.py

@@ -0,0 +1,3 @@
+print(<error descr="Unresolved reference '_T'">_T</error>, <error descr="Unresolved reference '_KT'">_KT</error>)
+print(__import__)
+print(len)

python/testSrc/com/jetbrains/python/PythonCompletionTest.java

@@ -1114,6 +1114,22 @@ public class PythonCompletionTest extends PyTestCase {
     assertSameElements(variants, "mod1", "mod2", "foo", "_bar");
   }
 
+  // PY-22828
+  public void testNoImportedBuiltinNames() {
+    final List<String> suggested = doTestByText("T<caret>\n");
+    assertNotNull(suggested);
+    assertContainsElements(suggested, "TypeError");
+    assertDoesntContain(suggested, "TypeVar");
+  }
+
+  // PY-22828
+  public void testNoProtectedBuiltinNames() {
+    final List<String> suggested = doTestByText("_<caret>\n");
+    assertNotNull(suggested);
+    assertContainsElements(suggested, "__import__");
+    assertDoesntContain(suggested, "_T", "_KT");
+  }
+
   @Override
   protected String getTestDataPath() {
     return super.getTestDataPath() + "/completion";

python/testSrc/com/jetbrains/python/inspections/PyUnresolvedReferencesInspectionTest.java

@@ -763,6 +763,11 @@ public class PyUnresolvedReferencesInspectionTest extends PyInspectionTestCase {
     doTest();
   }
 
+  // PY-22828
+  public void testNoProtectedBuiltinNames() {
+    doTest();
+  }
+
   @NotNull
   @Override
   protected Class<? extends PyInspection> getInspectionClass() {