The types exposed by the compose package are only used in the store API,
so move them there where they belong.
Making the ownership of the types clear, rather than having them live in
a package for themselves, will make it clearer how the types can be
modified in follow-up commits.
Also remove the JSON annotations, as these types are no longer used for
serialization.
Signed-off-by: Tom Gundersen <teg@jklm.no>