Reduce number of variables involved in resolution process

[m1kola]

Spinning out of #437

We need to explore if we can reduce number of variables involved in resolution.

Today we have the following variable types:

• RequiredPackageVariable - variable for what an user explicitly requested via Operator API. It contains constraints for potential candidates for installation.
• InstalledPackageVariable - variable for what we already have on the cluster. It contains constraints for potential successors of the currently installed bundles.
• BundleVariable - variable for each bundle involved in resolution. It contains constraints for depenencies of the bundle.
• BundleUniquenessVariable - variable that contains constraints to restrict the solution to at most a single bundle per package.

We need to look into whether we can combine some of the variables together.

Previous discussion on this: #460 (comment)

[m1kola]

No longer applicable since we removed Deppy