rest.tagpage plugin doesn't support swagger 2.0

**Operating System**: (`Windows` or `Linux` or `MacOS`)
All

**DocFX Version Used**:
2.57.1

**Template used**: (`default` or `statictoc` or contain custom template)
default

**Steps to Reproduce**:

1. Download the seed project from [docascode/docfx-seed](https://github.com/docascode/docfx-seed)
2. Install the rest.tagpage plugin and update docfx.json to use it
3. Build the documentation

**Expected Behavior**:
The tagpage plugin should work with both .json files

**Actual Behavior**:
The plugin only works with the petstore.swagger.json and not contacts_swagger2.json