This renames the backend packages to more closely align with the
new direction for them. Namely, pkg/backend/cloud becomes
pkg/backend/httpstate and pkg/backend/local becomes
pkg/backend/filestate. This also helps to clarify that these are meant
to be around state management and so the upcoming refactoring required
to split out (e.g.) the display logic (amongst other things) will make
more sense, and we'll need better package names for those too.
feaea31f7b