Improve versionmap.py

[rd4398]

#981 wraps the module as a provider. We need to improve the code in this module.

@tiran suggested following ideas:

• don't accept a str, only accept a Version. This would simplify the code and shift the responsibility of parsing a str as Version to the caller
• the new code needs version and url. Why not implement a function that yields version, url pairs?
• maybe change the VersionMap class to be a subclass of Mapping ABC?

@dhellmann added a comment to make API more tighter: #981 (comment)

We should do this improvement in a follow up MR.

[tiran]

I think you could also hard-code use_resolver_cache=False and remove cache_key.