Introduce xbps_{pkgdb,rpool}_get_pkg_fulldeptree().

These routines return a xbps_array_t with a full sorted dependency graph
for the target pkg, by querying pkgdb or rpool.

Update xbps-query(8) to use the new libxbps API.

lib/Makefile

@@ -39,7 +39,7 @@ OBJS += package_unpack.o package_register.o package_script.o verifysig.o
 OBJS += package_msg.o pkgdb_conversion.o transaction_shlibs.o
 OBJS += transaction_commit.o transaction_package_replace.o
 OBJS += transaction_dictionary.o transaction_ops.o transaction_store.o
-OBJS += transaction_revdeps.o pubkey2fp.o
+OBJS += transaction_revdeps.o pubkey2fp.o package_fulldeptree.o
 OBJS += download.o initend.o pkgdb.o package_conflicts.o
 OBJS += plist.o plist_find.o plist_match.o archive.o
 OBJS += plist_remove.o plist_fetch.o util.o util_hash.o