Give access to current repo

[Chris Warburton]

We can use names like nixpkgs1603 to access the pristine contents of
nixpkgs version 16.03, customised.nixpkgs1603 to access our overrides
applied to that version and repo1603 to access the source of nixpkgs
16.03.

These let us pick out things from particular nixpkgs versions,
regardless of what version we're overriding.

We can use super and self to get the (non-overridden and overridden,
respectively) versions of nixpkgs that we're currently overriding.

These let us maintain consistency within a nixpkgs version, and have
each version use its own packages in its overrides (e.g. so that the
conkeror in 17.03 will use the firefox in 17.03, and so on).

There doesn't seem to be a 'current' version (equivalent to self and
super) for the 'repoXXXX' case. This prevents us accessing files from
the current repo.

This would be useful, for example to load themes directly from their
.nix files.