Implemented a cache to get pkg dicts from metadir.

This improves xbps-pkgdb -a marginally, but still seems to be a bit slow.

lib/Makefile

@@ -44,7 +44,7 @@ EXTOBJS += external/mkpath.o
 OBJS = package_configure.o package_config_files.o package_orphans.o
 OBJS += package_remove.o package_remove_obsoletes.o package_state.o
 OBJS += package_unpack.o package_requiredby.o package_register.o
-OBJS += package_script.o
+OBJS += package_script.o package_metadir.o
 OBJS += transaction_commit.o transaction_package_replace.o
 OBJS += transaction_dictionary.o transaction_sortdeps.o transaction_ops.o
 OBJS += download.o initend.o pkgdb.o package_conflicts.o