Don't expose all functions in the top-level lib, prefer sub-libraries

[infinisil]

lib is a bit messy right now, where some functions are available in lib.*, but some need to be accessed as lib.<sublibrary>.*.

Traditionally all functions were available under lib.*, but in recent years we've seen that it doesn't work that well. An example is how lib.hasPrefix works on strings, it's aliased to lib.strings.hasPrefix. But it also makes sense to have such functionality for lists and paths, we should be able to re-use the same name. This is now done:

• lib.lists.{hasPrefix,removePrefix}: init #243511
• lib.path.hasPrefix: init #237610

But lib.hasPrefix still needs to point to lib.strings.hasPrefix for backwards compat, and this previously caused problems when accidentally using lib.hasPrefix for paths:

• lib.strings: Prevent paths as inputs in some functions #221204

Furthermore, lib is tending towards the style more emphasised since

• lib: Use Nix's static scope checking, fix error message, optimize #101139

{ lib }:
let
  inherit (lib.lists)
    length;

  inherit (lib.strings)
    hasPrefix;
in
...

which makes sublibraries not much harder to use (no need to always use lists.length).

We should establish a standard for sublibraries and make sure it's consistent.

Related issues/comments:

• lib: expose lib.cli namespace to lib #263969
• lib.strings: add replicate #261676 (comment)

Ping @roberth @h7x4