store: declare a `files` method on BaseStoreEntry
This will help pytype to type check. We have to move `StoreFile` earlier in the
file to use it in the type declaration.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}