tclPackages: init (draft)

[infinisil]

Motivated by this being requested on Discourse by @nat-418, this is a draft of how TCL packages could be handled.

Supported is:

• A tclPackages attribute containing the packages for the default tcl version, these attributes are built by Hydra and findable from nix search and co.
• tcl.pkgs, tcl-8_5.pkgs, tcl-8_6.pkgs containing package sets for the respective tcl version
• A tcl package scope, with a callPackage always passing the correct version of tcl, tclPackages, mkTclDerivation and all tcl packages attributes themselves.
• Ability to extend these package sets using .overrideScope' (self: super: { ... })
• Ability to extend every package set at once using config.tclPackagesExtensions = [ (self: super: { ... }) ]
• A directory pkgs/development/tcl-modules to group together tcl packages

@nat-418: I have no intention of pushing this PR over the finish line, but feel free to use the commits from this PR as a base for your own PR. At least some tests and documentation is still needed before this could be mergeable.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/a-tcl-package-set/28071/9