Moved the code that prunes the file list after reading the manifest

Moved the code that prunes the file list after reading the manifest
  template into a new method 'prune_file_list()', called from
  'get_file_list()' rather than 'read_manifest()' -- this keeps
  'read_manifest()' more general.
Deleted the redundant call to 'exclude_pattern()' in 'make_distribution()'
  -- this had the same intention as 'prune_file_list()', but was incomplete
  (only pruned the release tree, not the build tree) and in the wrong
  place (the prune wouldn't be reflected in the manifest file).