Refactor Ecosystem helper interface

[michaelkedar]

The Ecosystem class is confusing to use:

• It has abstract methods sort_key and enumerate_versions that are not required to be implemented (but must be overwritten with a function that raises a NotImplementedError)
• To check whether these are implemented, the respective (and poorly named) supports_comparing and supports_ordering properties must be manually implemented & checked.

In my opinion, it would be better to have a few interfaces:

• Ecosystem(), a replacement for the current OrderingUnsupportedEcosystem, with whatever base things are needed
• ComparableEcosystem(Ecosystem, ABC), which are ecosystems which we can create orderings of version strings (i.e. which implement sort_key)
• EnumerableEcosystem(ComparableEcosystem), which are ecosystems which we can find all concrete versions for (i.e. which implement enumerate_versions)

With this, we could replace the supports_x properties with isinstance calls. It also would make reusing logic for ecosystems which use the same versioning scheme but have different enumerations (e.g. APK/RPM ecosystems #3431)