Add implicit state file support

Read implicit package list from /home/rick/.config/declpac or ~/.config/declpac

README.md

@@ -57,6 +57,11 @@ docker
 # this is a comment
 ```
 
+### Implicit State File
+
+If `$XDG_CONFIG_HOME/declpac` (or `~/.config/declpac` on fallback) exists, its
+contents are automatically included in the package list.
+
 ### Options
 
 | Flag | Alias | Description |

pkg/input/input.go

@@ -31,6 +31,13 @@ func ReadPackages(stateFiles []string) (map[string]bool, error) {
 		}
 	}
 
+	implicitStateFile := getImplicitStateFile()
+	if fileExists(implicitStateFile) {
+		if err := readStateFile(implicitStateFile, packages); err != nil {
+			return nil, err
+		}
+	}
+
 	if err := readStdin(packages); err != nil {
 		return nil, err
 	}
@@ -52,6 +59,19 @@ func expandPath(path string) string {
 	return path
 }
 
+func fileExists(path string) bool {
+	_, err := os.Stat(path)
+	return err == nil
+}
+
+func getImplicitStateFile() string {
+	cfgDir, _ := os.UserConfigDir()
+	if cfgDir == "" {
+		cfgDir = "~/.config"
+	}
+	return filepath.Join(cfgDir, "declpac")
+}
+
 func normalizePackageName(name string) string {
 	name = strings.TrimSpace(name)
 	if name == "" || strings.HasPrefix(name, "#") {