repository refactoring + stubs.

repository::{builder,cache,distributor,extractor,local_system} stubs.
repository_error declaration and integration.
repository::distributor::create{,_recursively}() implementation.
repository::distributor::update_meta() implementation.
repository::distributor::update_sources() implementation.
{read,write}_snapshot() implementation.
{read,write}_checksum() implementation.
cache_config refactoring.
repository::cache partial implementation.

read_index()
read_checksum()
build_outdated()
installation_outdated()
checksum_path()
index_path()
build_archive_path()
build_snapshot_path()
installation_archive_path()
installation_snapshot_path()
repository::local_system implementation.
repository::builder implementation.