Refactor the setting to make unit test easier (#22405)

Some bugs caused by less unit tests in fundamental packages. This PR
refactor `setting` package so that create a unit test will be easier
than before.

- All `LoadFromXXX` files has been splited as two functions, one is
`InitProviderFromXXX` and `LoadCommonSettings`. The first functions will
only include the code to create or new a ini file. The second function
will load common settings.
- It also renames all functions in setting from `newXXXService` to
`loadXXXSetting` or `loadXXXFrom` to make the function name less
confusing.
- Move `XORMLog` to `SQLLog` because it's a better name for that.

Maybe we should finally move these `loadXXXSetting` into the `XXXInit`
function? Any idea?

---------

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: delvh <dev.lh@web.de>

modules/setting/config_provider.go

@@ -0,0 +1,39 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package setting
+
+import (
+	"code.gitea.io/gitea/modules/log"
+
+	ini "gopkg.in/ini.v1"
+)
+
+// ConfigProvider represents a config provider
+type ConfigProvider interface {
+	Section(section string) *ini.Section
+	NewSection(name string) (*ini.Section, error)
+	GetSection(name string) (*ini.Section, error)
+}
+
+// a file is an implementation ConfigProvider and other implementations are possible, i.e. from docker, k8s, …
+var _ ConfigProvider = &ini.File{}
+
+func mustMapSetting(rootCfg ConfigProvider, sectionName string, setting interface{}) {
+	if err := rootCfg.Section(sectionName).MapTo(setting); err != nil {
+		log.Fatal("Failed to map %s settings: %v", sectionName, err)
+	}
+}
+
+func deprecatedSetting(rootCfg ConfigProvider, oldSection, oldKey, newSection, newKey string) {
+	if rootCfg.Section(oldSection).HasKey(oldKey) {
+		log.Error("Deprecated fallback `[%s]` `%s` present. Use `[%s]` `%s` instead. This fallback will be removed in v1.19.0", oldSection, oldKey, newSection, newKey)
+	}
+}
+
+// deprecatedSettingDB add a hint that the configuration has been moved to database but still kept in app.ini
+func deprecatedSettingDB(rootCfg ConfigProvider, oldSection, oldKey string) {
+	if rootCfg.Section(oldSection).HasKey(oldKey) {
+		log.Error("Deprecated `[%s]` `%s` present which has been copied to database table sys_setting", oldSection, oldKey)
+	}
+}