fix(mfe): properly handle versionless configuration (#9945)
### Description
TSIA
We were requiring the version to exist in `VersionOnly` schema so we
could decide which config schema to use.
I'm assuming I broke this in
https://github.com/vercel/turborepo/pull/9883, but it's not immediately
obvious to me how.
### Testing Instructions
Added failing unit test for versionless `Config::load_from_dir` call
