Move "objects" to `object` dir

[carpawell]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I see a separate package per every non-regular object. E.g. tombstone, storagegroup, lock.

Describe the solution you'd like

All of them are the subparts of object pkg. So move it here.

Describe alternatives you've considered

Keep it as is. The number of packages is uncontrollable then.

Additional context

#283 (comment)

Also, I am not sure if it is good to make such changes, cause it may affect code generation (that also generates a package per object type).