Correctly resolve UCX/UCX_MO dependencies

[artpol84]

With the introduction of Multi-Object (MO) UCX backend (PR #58) that depends on UCX backend we need to make sure the dependency is properly handled for all static/dynamic plugin configurations.

Initial thoughts to consider:

• UCX is a plugin => UCX MO is a plugin and it just includes the code from UCX backend and not have plugin-to-plugin dependency
• UCX is static => 2 options
  • UCX_MO has to be static as well
  • UCX_MO can be whatever, but if it's a plugin:
    • Either it relies on libnixl for UCX backend functionality
    • OR we build UCX_MO plugin so such that UCX symbols are not conflicting with UCX backend code that is part of libnixl. This would require some investigation