Switch to extended Nixpkg's lib

This change makes use of the `extend` function inside `lib` to inject
a new `hm` field containing the Home Manager library functions. This
simplifies use of the Home Manager library in the modules and reduces
the risk of accidental infinite recursion.

PR #994

modules/files.nix

@@ -6,8 +6,6 @@ let
 
   cfg = config.home.file;
 
-  dag = config.lib.dag;
-
   homeDirectory = config.home.homeDirectory;
 
   fileType = (import lib/file-type.nix {
@@ -43,7 +41,7 @@ in
   config = {
     # This verifies that the links we are about to create will not
     # overwrite an existing file.
-    home.activation.checkLinkTargets = dag.entryBefore ["writeBoundary"] (
+    home.activation.checkLinkTargets = hm.dag.entryBefore ["writeBoundary"] (
       let
         check = pkgs.writeText "check" ''
           . ${./lib-bash/color-echo.sh}
@@ -113,7 +111,7 @@ in
     # and a failure during the intermediate state FA ∩ FB will not
     # result in lost links because this set of links are in both the
     # source and target generation.
-    home.activation.linkGeneration = dag.entryAfter ["writeBoundary"] (
+    home.activation.linkGeneration = hm.dag.entryAfter ["writeBoundary"] (
       let
         link = pkgs.writeText "link" ''
           newGenFiles="$1"
@@ -210,7 +208,7 @@ in
         ''
     );
 
-    home.activation.checkFilesChanged = dag.entryBefore ["linkGeneration"] (
+    home.activation.checkFilesChanged = hm.dag.entryBefore ["linkGeneration"] (
       ''
         declare -A changedFiles
       '' + concatMapStrings (v: ''
@@ -220,7 +218,7 @@ in
       '') (filter (v: v.onChange != "") (attrValues cfg))
     );
 
-    home.activation.onFilesChange = dag.entryAfter ["linkGeneration"] (
+    home.activation.onFilesChange = hm.dag.entryAfter ["linkGeneration"] (
       concatMapStrings (v: ''
         if [[ ${"$\{changedFiles"}["${v.target}"]} -eq 1 ]]; then
           ${v.onChange}