import-checker: allow all absolute imports of stdlib modules

Before this patch, we didn't allow imports like

  from importlib import resources

(That's the reason I used `import importlib.resources` in D7629.)

I think that form is still an absolute import, so I don't think we
forbade on purpose.

Differential Revision: https://phab.mercurial-scm.org/D7700

contrib/import-checker.py

@@ -536,7 +536,7 b' def verify_modern_convention(module, roo'
                 if not fullname or (
                     fullname in stdlib_modules
                     # allow standard 'from typing import ...' style
-                    and fullname != 'typing'
+                    and fullname.startswith('.')
                     and fullname not in localmods
                     and fullname + '.__init__' not in localmods
                 ):

tests/test-imports-checker.t

@@ -47,6 +47,11 b' Run additional tests for the import chec'
   > from .. import os
   > EOF
 
+  $ cat > testpackage/stdlibfrom.py << EOF
+  > from __future__ import absolute_import
+  > from collections import abc
+  > EOF
+
   $ cat > testpackage/symbolimport.py << EOF
   > from __future__ import absolute_import
   > from .unsorted import foo
@@ -150,6 +155,7 b' Run additional tests for the import chec'
   testpackage/requirerelative.py:2: import should be relative: testpackage.unsorted
   testpackage/sortedentries.py:2: imports from testpackage not lexically sorted: bar < foo
   testpackage/stdafterlocal.py:3: stdlib import "os" follows local import: testpackage
+  testpackage/stdlibfrom.py:2: direct symbol import abc from collections
   testpackage/subpackage/levelpriority.py:3: higher-level import should come first: testpackage
   testpackage/subpackage/localimport.py:7: multiple "from .. import" statements
   testpackage/subpackage/localimport.py:8: import should be relative: testpackage.subpackage.levelpriority