import-checker: make test-module-imports.t work using virtualenv (issue4129)

This patch modifies contrib/import-checker.py so that test-module-imports.t
will pass if run using virtualenv.  The patch achieves this by adding two
new prefixes to the list of allowable sys.path prefixes.  The added prefixes
are the directories of two modules in the stdlib.  The modules selected are
a minimal set that allowed the return value of list_stdlib_modules() to
match the return value without virtualenv, when run on the patch author's
machine: Mac OS X 10.8, Python 2.7.6.

contrib/import-checker.py

@@ -2,6 +2,12 @@ import ast
 import os
 import sys
 
+# Import a minimal set of stdlib modules needed for list_stdlib_modules()
+# to work when run from a virtualenv.  The modules were chosen empirically
+# so that the return value matches the return value without virtualenv.
+import BaseHTTPServer
+import zlib
+
 def dotted_name_of_path(path):
     """Given a relative path to a source file, return its dotted module name.
 
@@ -49,6 +55,21 @@ def list_stdlib_modules():
         yield m
     yield 'builtins' # python3 only
     stdlib_prefixes = set([sys.prefix, sys.exec_prefix])
+    # We need to supplement the list of prefixes for the search to work
+    # when run from within a virtualenv.
+    for mod in (BaseHTTPServer, zlib):
+        try:
+            # Not all module objects have a __file__ attribute.
+            filename = mod.__file__
+        except AttributeError:
+            continue
+        dirname = os.path.dirname(filename)
+        for prefix in stdlib_prefixes:
+            if dirname.startswith(prefix):
+                # Then this directory is redundant.
+                break
+        else:
+            stdlib_prefixes.add(dirname)
     for libpath in sys.path:
         # We want to walk everything in sys.path that starts with something
         # in stdlib_prefixes.