refactor: migrate db and config packages to use struct-based API

Removed global variables in favor of instance variables. This makes the code more maintainable and making it easier to write unit tests without relying on global state.

Marked the old functions with global state as obsolete, redirecting them to use a new API based on struct in order to rewrite the code using these functions gradually.

internal/config/config_legacy.go

@@ -0,0 +1,65 @@
+/*
+ * ALR - Any Linux Repository
+ * Copyright (C) 2024 Евгений Храмов
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package config
+
+import (
+	"context"
+	"sync"
+
+	"plemya-x.ru/alr/internal/types"
+)
+
+var defaultConfig = &types.Config{
+	RootCmd:          "sudo",
+	PagerStyle:       "native",
+	IgnorePkgUpdates: []string{},
+	Repos: []types.Repo{
+		{
+			Name: "default",
+			URL:  "https://gitea.plemya-x.ru/xpamych/xpamych-alr-repo.git",
+		},
+	},
+}
+
+// Config returns a ALR configuration struct.
+// The first time it's called, it'll load the config from a file.
+// Subsequent calls will just return the same value.
+//
+// Deprecated: use struct method
+func Config(ctx context.Context) *types.Config {
+	return GetInstance(ctx).cfg
+}
+
+// =======================
+// FOR LEGACY ONLY
+// =======================
+
+var (
+	alrConfig     *ALRConfig
+	alrConfigOnce sync.Once
+)
+
+func GetInstance(ctx context.Context) *ALRConfig {
+	alrConfigOnce.Do(func() {
+		alrConfig = New()
+		alrConfig.Load(ctx)
+	})
+
+	return alrConfig
+}