Sort studies by name and add basic study intersection check [prod]. (#1023)

[goodov]

Cherry-pick of https://github.com/brave/brave-variations/pull/1023

Copied from the original PR: This is a required change to migrate to per-file study structure, because per-file structure implies study enumeration in alphabetical order (as files lay on the file system).

This change should be a no-op as long as we don't have studies intersection. This PR adds the required validation to ensure we do not have this kind of intersection, which means if the seed is validated, then it doesn't depend on the initial order of studies in seed.json and it's safe to reorder studies.

We need to have the updated seed to work so in future we can switch the underlying generator without worrying too much.