[docs-verifier] Redirection verifier needs to handle multiple redirection files

[gewarren]

1. Needs to account for possibility of multiple redirection files. All redirection files (if there is more than one) will be listed in the .openpublishing.publish.config.json file like so:

"docsets_to_publish": [
    {
      ...
      "build_entry_point": "docs",
      "redirection_files": [
        ".openpublishing.redirection.json",
        ".openpublishing.redirection.architecture.json",
        ".openpublishing.redirection.azure.json",
        ".openpublishing.redirection.core.json"
      ],
      ...
    }

2. For redirects that use source_path (as opposed to source_path_from_root), the path to the file is relative to the location of the redirection file that contains it. So we need to check the path a little differently for this one.

[gewarren]

I'm currently working on no. 1.

[gewarren]

Redirection files no longer need to be listed in openpublishing.config.json. They are just found dynamically. And I noticed in this PR that a new redirection file isn't being picked up by docs-verifier, and so it's giving a false positive "no redirection added".