To make an index-related FSFS private API feasible, we need to publish
svn_fs_fs__p2l_entry_t and its substructure svn_fs_fs__id_part_t as
part of the private API.

* subversion/include/private/svn_fs_fs_private.h
  (svn_fs_fs__id_part_t,
   svn_fs_fs__p2l_entry_t): Move definitions here.

* subversion/libsvn_fs_fs/id.h
  (svn_fs_fs__id_part_t): Remove here.

* subversion/libsvn_fs_fs/index.h
  (svn_fs_fs__p2l_entry_t): Remove here.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632856 13f79535-47bb-0310-9956-ffa450edef68