Require compat statements for JLLs.

[vchuravy]

In #516 we discussed requiring compat bounds for JLLs.

It is probably something we should do, but may also be hard to pull off
since we need to automatically add compat bounds to the registry.

Maybe a step forward would be to record if a JLL doesn't follow semver?
Then we could make decision based on that instead of asuming that all JLLs don't follow semver.

cc: @giordano @staticfloat