support for lockfiles

[teto]

Though it's quite rare to see in the wild, luarocks supports lockfiles:
https://github.com/luarocks/luarocks/wiki/Pinning-versions-with-a-lock-file

Only example I know is:
https://github.com/luarocks/luarocks-site/blob/master/luarocks.lock

this is a lua file but simple enough that it can be parsed robustly (at least more easily than the rockspec).
This means it could be parsed directly via nix and in the case we are interested in fetching only dependencies (like in neorocks). The interesting aspect of this for luarocks-nix is to pin dependencies but it could be interesting for nixpkgs too.

cc @mrcjkb

[Freed-Wu]

upstream luarocks have support --force-lock. luarocks-nix still doesn't support.

[mrcjkb]

I played around with luarocks lockfiles a bit in nvim-neorocks/rocks.nvim#244 and found the feature quite lacking, clumsy and buggy.
I PRd a fix for a bug I encountered, but that still hasn't been released.

That's why in rocks, we've implemented cargo style lockfiles that include rockspec + source hashes, which should hopefully one day be usable with nix 😉