feat: Breaking out more dependencies.

Sources/ConfigurationClient/Constants.swift

@@ -0,0 +1,28 @@
+@_spi(Internal)
+public enum EnvironmentKey {
+  static let xdgConfigHome = "XDG_CONFIG_HOME"
+  static let hpaConfigHome = "HPA_CONFIG_HOME"
+  static let hpaConfigFile = "HPA_CONFIG_FILE"
+}
+
+@_spi(Internal)
+public enum HPAKey {
+  public static let configDirName = "hpa"
+  public static let resourceFileName = "hpa"
+  public static let resourceFileExtension = "toml"
+  public static let defaultFileName = "config.toml"
+}
+
+extension [String: String] {
+  var xdgConfigHome: String {
+    self[EnvironmentKey.xdgConfigHome] ?? "~/.config"
+  }
+
+  var hpaConfigHome: String? {
+    self[EnvironmentKey.hpaConfigHome]
+  }
+
+  var hpaConfigFile: String? {
+    self[EnvironmentKey.hpaConfigFile]
+  }
+}