pacman/sync: rewrite target handling

pacman/sync: rewrite target handling

This uses the new public functions to handle targets from the frontend,
like it used to be :
1) alpm_find_dbs_satisfier to find (optionally versioned) package or
provision
2) alpm_find_grp_pkgs to find members for a groups
3) alpm_add_pkg to finally add the pmpkg_t from 1 or 2

Of course, this adds more code to the frontend, but it completely
deprecates sync_target and sync_dbtarget interfaces.

This all-in-one interfaces felt wrong and left no control to the
frontend. A good frontend should just use alpm_add_pkg, with pkg coming
from alpm_db_get_pkg (for normal targets), alpm_find_dbs_satisfier (for
versioned provisions) or alpm_find_grp_pkgs (for groups).

This also opens the way to provide a better group handling in pacman
without constraint from libalpm and callbacks.

In ignore006, only the retcode changes, because no package was found to
satisfy the target (the only possible package is ignored).

Signed-off-by: Xavier Chantry <chantry.xavier@gmail.com>