The RepoContext encapsulates the list of repositories, as well as some
associated state. In particular, it also encapsulates the HttpTransport, which
will be initialized on demand and cached thereafter.  This is important for two
reasons:

* For the hackage-security integration: in order to be able to use cabal's own
  HttpTransport API for the secure repo, we need to have access to that
  transport when we initialize the repo, but as things stood that was not
  possible (cabal was initializing repos ahead of time but the transport on
  demand).

* For the integration with the nix-local-branch it is important that the Repo
  type remains Serializable. By passing RepoContext rather than a list of
  Repos, we can leave RepoSecure serializable and separately maintain a mapping
  from cabal's Repo type to hackage-security's (stateful) Repository type.