lib: Add readTree function to filesystem
lib: Add readTree function to filesystem

Add a friendly function to easily return a flattened list of files
within a directory.

This is useful if you want to easily iterate or concatSep the list of
files all found within a directory.
(i.e. when constructing Java's CLASSPATH)

Style improvements

Co-authored-by: Silvan Mosberger <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@infinisil.com>

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs