Refactor "flex-list" to "flex-divided-list" (#37505)

Purpose:

1. Make the whole code base have unified "item" layout
2. Clarify our "list" styles: "flex-relaxed-list", "flex-divided-list"
3. Prepare to replace legacy "ui relaxed list"
* https://github.com/go-gitea/gitea/pull/37445#discussion_r3144458865
4. Prepare for refactoring the "pull merge box", it needs the
"flex-divided-list"
    * related to "Refactor pull request view (*)" like #37451
5. Fix legacy abuses of "flex-list", e.g.: repo home sidebar

templates/package/shared/list.tmpl

@@ -16,14 +16,14 @@
 {{end}}
 <div>
 	{{range .PackageDescriptors}}
-	<div class="flex-list">
-		<div class="flex-item">
-			<div class="flex-item-main">
-				<div class="flex-item-title">
+	<div class="flex-divided-list items-with-main">
+		<div class="item">
+			<div class="item-main">
+				<div class="item-title">
 					<a href="{{.VersionWebLink}}">{{.Package.Name}}</a>
 					<span class="ui label">{{svg .Package.Type.SVGName 16}} {{.Package.Type.Name}}</span>
 				</div>
-				<div class="flex-item-body">
+				<div class="item-body">
 					{{$timeStr := DateUtils.TimeSince .Version.CreatedUnix}}
 					{{$hasRepositoryAccess := false}}
 					{{if .Repository}}