move moby components that do not have runtime dependencies to own directory

[deitch]

- What I did

The source directory src/cmd/linuxkit/moby/ contains all of the following:

• types
• utility functions (mostly related to those types)
• complex functions, those that actually build images, or convert them to/from formats, etc.

This means that anything that accesses or imports the types or utility functions also imports the dependencies of the complex functions, which means pulling in docker, etc., with all of the dependency management that implies.

This PR moves the first two into src/cmd/linuxkit/moby/util/, so it is possible to import those without all of the rest.

In retrospect, I easily could have done the reverse: leave those in moby/ and move the complex ones into moby/build/ or similar, but this worked fine.

- How I did it

Created a directory, moved files over, figured out vet issues, fixed them.

- How to verify it

CI. No change in functionality, so either it works or it doesn't.

- Description for the changelog

Internal split of functionality between packages for moby.

[deitch]

OK, I changed my mind and reversed it. Types are in moby/ and build functions (that depend on them) are in moby/build/