Add support for declaring optional dependencies for component aliases

Currently the component aliases may declare hard dependencies to components or other aliases - if the required dependency is missing, the alias that declares the dependency is marked unstable.

For making the alias declaration more flexible, support for optional dependencies should be added. This enables better structuring of alias dependencies, where some requires aliases may be available for only certain users, licenses, etc. but installation of the dependent alias should not be blocked in case the dependency is not available.