Tags do not work with flat content layout

[yuri1969]

Description

The tags path /tags/ introduced in 709a407 does not work on a project using a flat content layout with no docs/ nor blog/ dirs.

Example flat layout:

content/
├── foo
│   └── _index.md
├── bar
│   └── _index.md
└── _index.md

With the root _index.md file featuring the following header to set the docs/ behavior to all sub-dirs:

---
cascade:
  type: docs
---

PS: This layout is used due using just the docs/ behavior with no intention of introducing the blog/ behavior.

Steps To Reproduce

1. Use Hextra v0.9.3
2. Setup a content layout described above
3. hugo serve
4. Navigate to localhost:1313/tags/
5. The tags page is the same as pre-0.9.3

Expected Behavior

The /tags/ page should feature a single centered header as defined by 0.9.3.

Actual Behavior

The /tags/ page features just the default layout with a duplicated Tags header.

Screenshots

Environment

• Hugo Version: v0.127.0
• Browser/OS: Firefox
• Theme Version: v0.9.3