Remove `@oclif/core` dependency / change to peer dependency

[dsokal]

Currently, @oclif/core depends on cli-ux which depends on @oclif/core, and which depends on cli-ux, and so on.
This cycle can cause (and actually causes) multiple versions of @oclif/core and cli-ux exist in node_modules.

There are a few solutions to this problem, e.g.:

• You could add @oclif/core as a peer dependency.
• You could extract the common functionality to a separate package.

[RodEsp]

Removing circular dependencies is certainly something we've been working on recently. Have done this for a few other repos already and will eventually get to these at some point too. Thanks for pointing it out!