Solver: Introduce constraints for "upgrade edges"

[jmprusi]

For an operator upgrade to be valid, the new operator version should comply with a set of constraints:

• Channel: The new operator version should be part of the same channel
• Replaces: The currently installed operator version should be included in the "replace" field of the new desired operator version (see skips)
• Skips: Skips define from what version it is possible to jump to without upgrading one version at a time. This should be considered and not only the "replaces" field.

related to #231

[joelanford]

Channel: The new operator version should be part of the same channel

This is a anti-goal. This would prohibit upgrades between channels even when there is an upgrade edge from the currently installed version defined in the new channel (and the user is not opposed to using that channel)

[joelanford]

Related: #274 (comment)