refactor: move support functions to ./lib; change piholeConfiguration option to piholeConfig

The first part of the refactor simplifies the logic in the module and hides functions which are only used to extract container env vars.
Second the option renaming to piholeConfig unifies the naming with the hostConfig option.

lib/util.nix

@@ -1,5 +1,5 @@
-{
-  collectAttrFragments = predicate: attrs: with builtins; let
+with builtins; let
+  collectAttrFragments = predicate: attrs: let
     _collectAttrFragments = attrs:
       concatMap (key: _collectAttrFragmentsBelowKey key attrs.${key}) (attrNames attrs)
     ;
@@ -12,7 +12,7 @@
     in _collectAttrFragments attrs
   ;
 
-  accessValueOfFragment = attrs: fragment: with builtins; let
+  accessValueOfFragment = attrs: fragment: let
     _accessValueOfFragment = value: fragment:
       if fragment == [] then value
       else _accessValueOfFragment (value.${head fragment}) (tail fragment)
@@ -20,8 +20,26 @@
     in _accessValueOfFragment attrs fragment
   ;
 
-  toEnvValue = value: with builtins;
-  if isBool value then (if value then "true" else "false")
-  else if isList value then "[${concatStringSep ";" value}]"
-  else value;
+  toEnvValue = value:
+    if isBool value then (if value then "true" else "false")
+    else if isList value then "[${concatStringSep ";" value}]"
+    else value
+  ;
+
+in {
+  extractContainerEnvVars = piholeOptionDeclarations: piholeOptionDefinitions: let
+    _opt = piholeOptionDeclarations;
+    _cfg = piholeOptionDefinitions;
+
+    _envVarFragments = collectAttrFragments (value: isAttrs value && value ? "envVar") _opt.piholeConfig;
+    in filter
+      (envVar: envVar.value != null)
+      (map
+        (fragment: {
+          name = getAttr "envVar" (accessValueOfFragment _opt.piholeConfig fragment);
+          value = toEnvValue (accessValueOfFragment _cfg.piholeConfig fragment);
+        })
+        _envVarFragments
+      )
+  ;
 }