Ambiguities in lts versioning

[4e6]

The last update revealed that current naming can be ambiguous. There is a conflict between lts-1.10 and lts-11.0 which result to lts-110 Nix key, see stackage/default.nix

Nix doesn't allow dots in the attribute keys. Removing the dots, the standard Nix way to deal with that seems doesn't work in our case.

Come up with a different naming scheme.

[dhess]

Nixpkgs generally uses underscore (_) to replace . in package names that include versions, so one idea that's consistent with Nixpkgs would be, e.g., lts-11_0 and lts-1_10.

[4e6]

As far as I remember, back in the days, Nixpkgs used this particular naming for Stackage LTS releases, so I decided to inherit it.

Sounds good. Let's stick to the underscore character instead of the dots since it is the established naming.