Refactor directory structure

Benefits of this refactoring:
- argument list is now comprehensibly as there is now only one nixpkgs
  version flying around
- common config and lib functions got abstracted away
- clearer intent when to use pinned or unpinned packages
- allows better configurability for the future through config attr set

default.nix

@@ -13,7 +13,7 @@ let
 in
 
 {
-  inherit (pkgs) proot;
+  inherit (pkgs) proot-termux;
 
   basic-environment = nixpkgs.buildEnv {
     name = "basic-environment";
@@ -23,7 +23,7 @@ in
       nixpkgs.cacert
       nixpkgs.coreutils
       nixpkgs.nix
-      # pkgs.proot
+      # pkgs.proot-termux
       pkgs.files.etc-group
       pkgs.files.etc-passwd
       pkgs.files.hm-install