Drop egg_info caches after cleaning requirements

After updating requirement lists with forced requirements we should
re-run egg-info to get updated data from them, because using stale data
from caches makes requirements cleaning effectively useless.

In this change we:
- drop _egg_info member from PythonInstallComponent, as we don't need
  two levels of caching;
- add drop_caches method to pip_helper to clean it's caching;
- add call to pip_helper.drop_caches() after cleaning requirements.

Closes-bug: #1250926
Change-Id: I670d4f99cf585f722a3c0600bb40bffe1f217de0