Add build file generation override for cncf/xds (#2027)

**What type of PR is this?**

Other

**What package or component does this PR mostly affect?**

internal/bzlmod

**What does this PR do? Why is it needed?**

Enable build file generation on a subpath to work around
https://github.com/cncf/xds/issues/104.

**Which issues(s) does this PR fix?**

**Other notes for review**