import-checker: refactor sys.path prefix check (issue4129)

This patch refactors the logic in contrib/import-checker.py responsible for
checking the beginnings of the paths in sys.path.  In particular, it adds a
variable that defines the set of allowed prefixes.

The primary purpose of this change is to make it easier to add more allowed
prefixes.  This will be useful in resolving issue4129, which involves making
the function list_stdlib_modules() work when run from a virtualenv.

contrib/import-checker.py

@@ -48,11 +48,14 b' def list_stdlib_modules():'
     for m in 'ctypes', 'email':
         yield m
     yield 'builtins' # python3 only
+    stdlib_prefixes = set([sys.prefix, sys.exec_prefix])
     for libpath in sys.path:
-        # We want to walk everything in sys.path that starts with
-        # either sys.prefix or sys.exec_prefix.
-        if not (libpath.startswith(sys.prefix)
-                or libpath.startswith(sys.exec_prefix)):
+        # We want to walk everything in sys.path that starts with something
+        # in stdlib_prefixes.
+        for prefix in stdlib_prefixes:
+            if libpath.startswith(prefix):
+                break
+        else:
             continue
         if 'site-packages' in libpath:
             continue