Reduce number of resolution variables to one per bundle

[m1kola]

Today one bundle in the catalog may result in multiple variables for the solver due to the fact that we create one variable per bundle occurrence in each channel. For example, If package foo has bundle foo.v1 which occurs in channels beta and stable, we create two variables for the solver.

This is the case currently in main branch where we create entities (which later get translated into variables) here and in the upcoming PR which replaces entities & entity sources (see this discussion).

This issue is to look into whether we can reduce a number of variables we create to one per bundle.

This should be possible because we should be doing all the filtering of bundles by channel before we create a variable for the solver.