PY-25234 More efficient way to check that a user skeleton belongs to the stdlib

Also, I added a test on that scenario using the skeleton for "alembic"
as an example.

python/src/com/jetbrains/python/codeInsight/userSkeletons/PyUserSkeletonsUtil.java

@@ -15,12 +15,13 @@
  */
 package com.jetbrains.python.codeInsight.userSkeletons;
 
-import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
 import com.intellij.openapi.application.PathManager;
 import com.intellij.openapi.diagnostic.Logger;
 import com.intellij.openapi.project.Project;
 import com.intellij.openapi.projectRoots.Sdk;
 import com.intellij.openapi.util.Key;
+import com.intellij.openapi.util.text.StringUtil;
 import com.intellij.openapi.vfs.StandardFileSystems;
 import com.intellij.openapi.vfs.VfsUtilCore;
 import com.intellij.openapi.vfs.VirtualFile;
@@ -56,7 +57,7 @@ public class PyUserSkeletonsUtil {
   private static final Logger LOG = Logger.getInstance("#com.jetbrains.python.codeInsight.userSkeletons.PyUserSkeletonsUtil");
   public static final Key<Boolean> HAS_SKELETON = Key.create("PyUserSkeleton.hasSkeleton");
 
-  private static final ImmutableList<String> STDLIB_SKELETONS = ImmutableList.of(
+  private static final ImmutableSet<String> STDLIB_SKELETONS = ImmutableSet.of(
     "asyncio",
     "multiprocessing",
     "os",
@@ -130,12 +131,13 @@ public class PyUserSkeletonsUtil {
     if (skeletonsDir == null) {
       return false;
     }
-    final String relativePath = VfsUtilCore.getRelativePath(virtualFile, skeletonsDir);
+    final String relativePath = VfsUtilCore.getRelativePath(virtualFile, skeletonsDir, '/');
     // not under skeletons directory
     if (relativePath == null) {
       return false;
     }
-    return ContainerUtil.exists(STDLIB_SKELETONS, relativePath::startsWith);
+    final String firstComponent = ContainerUtil.getFirstItem(StringUtil.split(relativePath, "/"));
+    return STDLIB_SKELETONS.contains(firstComponent);
   }
 
   @Nullable

python/testData/quickFixes/PyAddImportQuickFixTest/userSkeletonStdlibModule/main.py

@@ -0,0 +1,3 @@
+import sys
+
+print(<error descr="Unresolved reference 'alembic'">alem<caret>bic</error>, sys)

python/testData/quickFixes/PyAddImportQuickFixTest/userSkeletonStdlibModule/main_after.py

@@ -0,0 +1,5 @@
+import sys
+
+import alembic
+
+print(alembic, sys)

python/testSrc/com/jetbrains/python/quickFixes/PyAddImportQuickFixTest.java

@@ -55,10 +55,16 @@ public class PyAddImportQuickFixTest extends PyQuickFixTestCase {
     doMultiFileAutoImportTest("Import this name");
   }
 
-  public void testSkeletonStdlibModule() {
+  // PY-25234
+  public void testBinarySkeletonStdlibModule() {
     doMultiFileAutoImportTest("Import 'sys'");
   }
 
+  // PY-25234
+  public void testUserSkeletonStdlibModule() {
+    doMultiFileAutoImportTest("Import 'alembic'");
+  }
+
   private void doMultiFileAutoImportTest(@NotNull String hintPrefix) {
     myFixture.copyDirectoryToProject(getTestName(true), "");
     myFixture.enableInspections(PyUnresolvedReferencesInspection.class);