Previously in v1alpha1, all Holos structs are located in the same
package. This makes it difficult to focus on only the structs necessary
to transfer configuration data from CUE to the `holos` cli.
This patch splits the structs into `meta` and `core` where the core
package holds the structs end users should refer to and focus on. Only
the Platform resource is in core now, but other BuildPlan types will be
added shortly.
