import-checker: report local with stdlib late warning

Without this, developers have to figure it out on their own

contrib/import-checker.py

@@ -366,7 +366,7 b' def verify_modern_convention(module, roo'
     fromlocal = fromlocalfunc(module, localmods)
 
     # Whether a local/non-stdlib import has been performed.
-    seenlocal = False
+    seenlocal = None
     # Whether a relative, non-symbol import has been seen.
     seennonsymbolrelative = False
     # The last name to be imported (for sorting).
@@ -403,10 +403,11 b' def verify_modern_convention(module, roo'
             # stdlib imports should be before local imports.
             stdlib = name in stdlib_modules
             if stdlib and seenlocal and node.col_offset == root_col_offset:
-                yield msg('stdlib import follows local import: %s', name)
+                yield msg('stdlib import "%s" follows local import: %s',
+                          name, seenlocal)
 
             if not stdlib:
-                seenlocal = True
+                seenlocal = name
 
             # Import of sibling modules should use relative imports.
             topname = name.split('.')[0]
@@ -437,7 +438,7 b' def verify_modern_convention(module, roo'
                 if not fullname or fullname in stdlib_modules:
                     yield msg('relative import of stdlib module')
                 else:
-                    seenlocal = True
+                    seenlocal = fullname
 
             # Direct symbol import is only allowed from certain modules and
             # must occur before non-symbol imports.

tests/test-check-module-imports.t

@@ -125,7 +125,7 b' Run additional tests for the import chec'
   testpackage/relativestdlib.py:2: relative import of stdlib module
   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 follows local import: os
+  testpackage/stdafterlocal.py:3: stdlib import "os" follows local import: testpackage
   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