bazel: difficult to get `nogo` to ignore all third-party code

[rickystewart]

To ignore certain files in nogo, we need to pass in a regex to match the filename as described here. However, there is no one regex that allows us to reliably recognize all third-party code and match no first-party code. Ideally, this should be a "meta" thing where nogo can simply check what is or is not in external and allow configuring that directly.

Epic: CRDB-17171

Jira issue: CRDB-38733