import-checker: make stdlib path detection work in virtual environments

The previous logic tried to find the directory containing BaseHTTPServer, which
didn’t work as indended because it was only present on Python 2. Instead, the
argparse module is used now.

contrib/import-checker.py

@@ -11,10 +11,7 b' import sys'
 # to work when run from a virtualenv.  The modules were chosen empirically
 # so that the return value matches the return value without virtualenv.
 if True:  # disable lexical sorting checks
-    try:
-        import BaseHTTPServer as basehttpserver
-    except ImportError:
-        basehttpserver = None
+    import argparse
     import zlib
 
 import testparseutil
@@ -244,7 +241,7 b' def list_stdlib_modules():'
     stdlib_prefixes = {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):
+    for mod in (argparse, zlib):
         if mod is None:
             continue
         try: