[Bug]: group in docfx.json can not work when building the docfx.json with multi versions

**Describe the bug**
When I run the docfx build docfx.json, there are some warnings:
```
D:\codetest\docfx_project\~\api\v1\toc.yml: warning UidNotFound: Unable to find file with uid "LibraryTest" referenced by TOC file "~/api/v1/toc.yml"
D:\codetest\docfx_project\~\api\v1\toc.yml: warning UidNotFound: Unable to find file with uid "LibraryTest.Class1" referenced by TOC file "~/api/v1/toc.yml"
D:\codetest\docfx_project\~\api\v1\toc.yml: warning UidNotFound: Unable to find file with uid "LibraryTest.IMove" referenced by TOC file "~/api/v1/toc.yml"
D:\codetest\docfx_project\~\api\v1\toc.yml: warning UidNotFound: Unable to find file with uid "LibraryTest.People" referenced by TOC file "~/api/v1/toc.yml"
D:\codetest\docfx_project\~\api\v2\toc.yml: warning UidNotFound: Unable to find file with uid "LibraryTest" referenced by TOC file "~/api/v2/toc.yml"
D:\codetest\docfx_project\~\api\v2\toc.yml: warning UidNotFound: Unable to find file with uid "LibraryTest.Class1" referenced by TOC file "~/api/v2/toc.yml"
D:\codetest\docfx_project\~\api\v2\toc.yml: warning UidNotFound: Unable to find file with uid "LibraryTest.IMove" referenced by TOC file "~/api/v2/toc.yml"
D:\codetest\docfx_project\~\api\v2\toc.yml: warning UidNotFound: Unable to find file with uid "LibraryTest.People" referenced by TOC file "~/api/v2/toc.yml"
```
And I can not link to the versioned conceptual sdk document.

Here is my [project](https://github.com/jeffhli/docfx-test).

**Expected behavior**
It could generate 2 versions document.

**Context (please complete the following information):**
- OS: Windows
- Docfx version: [e.g. 2.71.0]

https://github.com/dotnet/docfx/issues/5252#issuecomment-799675484