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

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.