Path library functions

[infinisil]

For something like source combinators we should have a solid foundation for functions interacting with path values. This issue tracks progress on such a path library.

Originally such a path library was proposed in #200718, but has been redesigned after feedback from @roberth, @fricklerhandwerk and taking (reasonable) lazy tree-compatibility into account.

• General design, subpath validation and normalisation: lib.path.subpath.{isValid,normalise}: init #205190
• Appending subpaths: lib.path.append: init #208887
• Joining subpaths together: lib.path.subpath.join: init #209099
• Deprecating confusing string functions with paths: lib.strings: Prevent paths as inputs in some functions #221204
• Path ordering: lib.path.hasPrefix: init #237610
• Prefix stripping: lib.path.removePrefix: init #238013
• Path component common prefix: lib.lists.{findFirstIndex,commonPrefix}: init #243520
• Path parts splitting: lib.path.splitRoot: init #244358
• Subpath component splitting: lib.path.subpath.components: init #242695
• Path component list prefix checking and removal: lib.lists.{hasPrefix,removePrefix}: init #243511
• Path difference and common ancestor: lib.path.difference: init #209375
• Path ordering and prefix stripping: lib.path.hasPrefix, lib.path.removePrefix: init #210423
• Subpath-based source filtering: [WIP] lib.sources.predicateFilter: init #221361

There's also more functions we could add:

• Re-exporting safer versions of dirOf and baseNameOf from lib.path

[nixos-discourse]

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

https://discourse.nixos.org/t/tweag-nix-dev-update-47/27387/1

[infinisil]

Closing this because it is done, see https://nixos.org/manual/nixpkgs/unstable/#sec-functions-library-path!