tools.lazy-store: parse narinfos and store paths with harmonia

closure uses harmonia-store-nar-info's parse_narinfo_txt and
harmonia-store-path's StorePath instead of our key/value narinfo map
and string splitting: the closure walk, referrers and missing sets are
keyed by StorePath (whose order is the string order, so paths.json and
db.dump come out the same), references, deriver, NarHash, NarSize,
URL, Compression and FileSize come from the parsed NarInfo. A narinfo
that doesn't parse counts as a failed fetch from that cache (try the
next one) instead of failing the whole run later on a missing field.
Roots that aren't store paths are an error.

paths.json, db.dump, narinfo/ and closure.json are byte-identical to
before for the closures of hello and git from cache.nixos.org.

The fetch test's path without a narinfo used an "e" hash, which isn't
nix32 and so no store path.