plugin/none-ls: Implement all builtins (nix-community#1169)

* plugins/none-ls: Use upstream builtins.json to list sources

This avoids the need to define sources in nixvim. We only need to define
a mapping from source names to packages.

This commit does not yet introduce sources for the newly available builtins

* plugin/none-ls: Enable all known sources in tests

* plugins/none-ls: Add all missing sources