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/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)