docs: def clean_source_index(self):
          """
          Cleanup broken symbolic links in the local source distribution index.
  
          The purpose of this method requires some context to understand. Let me
          preface this by stating that I realize I'm probably overcomplicating
          things, but I like to preserve forward / backward compatibility when
          possible and I don't feel like dropping everyone's locally cached
          source distribution archives without a good reason to do so. With that
          out of the way:
  
          - Versions of pip-accel based on pip 1.4.x maintained a local source
            distribution index based on a directory containing symbolic links
            pointing directly into pip's download cache. When files were removed
            from pip's download cache, broken symbolic links remained in
            pip-accel's local source distribution index directory. This resulted
            in very confusing error messages. To avoid this
            :func:`clean_source_index()` cleaned up broken symbolic links
            whenever pip-accel was about to invoke pip.
  
          - More recent versions of pip (6.x) no longer support the same style of
            download cache that contains source distribution archives that can be
            re-used directly by pip-accel. To cope with the changes in pip 6.x
            new versions of pip-accel tell pip to download source distribution
            archives directly into the local source distribution index directory
            maintained by pip-accel.
  
          - It is very reasonable for users of pip-accel to have multiple
            versions of pip-accel installed on their system (imagine a dozen
            Python virtual environments that won't all be updated at the same
            time; this is the situation I always find myself in :-). These
            versions of pip-accel will be sharing the same local source
            distribution index directory.
  
          - All of this leads up to the local source distribution index directory
            containing a mixture of symbolic links and regular files with no
            obvious way to atomically and gracefully upgrade the local source
            distribution index directory while avoiding fights between old and
            new versions of pip-accel :-).
  
          - I could of course switch to storing the new local source distribution
            index in a differently named directory (avoiding potential conflicts
            between multiple versions of pip-accel) but then I would have to
            introduce a new configuration option, otherwise everyone who has
            configured pip-accel to store its source index in a non-default
            location could still be bitten by compatibility issues.
  
          For now I've decided to keep using the same directory for the local
          source distribution index and to keep cleaning up broken symbolic
          links. This enables cooperating between old and new versions of
          pip-accel and avoids trashing user's local source distribution indexes.
          The main disadvantage is that pip-accel is still required to clean up
          broken symbolic links...
          """
          cleanup_timer = Timer()
          cleanup_counter = 0
          for entry in os.listdir(self.config.source_index):
              pathname = os.path.join(self.config.source_index, entry)
              if os.path.islink(pathname) and not os.path.exists(pathname):
                  logger.warn("Cleaning up broken symbolic link: %s", pathname)
                  os.unlink(pathname)
                  cleanup_counter += 1
          logger.debug("Cleaned up %i broken symbolic links from source index in %s.", cleanup_counter, cleanup_timer)