This patch uses sys.path_importer_cache to reduce the
number of open calls, in the following way:
- if the value in path_importer_cache is None, it stats
the path to find out whether the file exists
- it then puts True/False into path_importer_cache
- if the value in path_importer_cache is False, the
path entry is skipped on all imports
- if the value is True, the stat call is skipped, and
open calls for files in the directory are made.

On Linux, this reduces the number of open calls for an
empty script from 343 to 263. The startup-time (for 100
interpreter invocations) goes down by one percent (from
0.0819s to 0.08113s per invocation).