Taskgraph fails with non-standard root dir

[ahal]

Taskgraph supports a --root flag which supposedly allows you to place the Taskgraph files into an arbitrary directory. However, due to this check:
https://github.com/taskcluster/taskgraph/blob/main/src/taskgraph/config.py#L130

Doing so results in a guaranteed exception. I'd be shocked if this were the only issue. We should either support this use case or remove the flag. This came up in https://github.com/mozilla/neqo because @larseggert would like to keep the CI files in the .taskcluster/ dir to help keep them hidden from the rest of the repo files.