Allow switching out the resolver's IO (#517)
I'm working off of @konstin's commit here to implement arbitrary unsat
test cases for the resolver.
The entirety of the resolver's io are two functions: Get the version map
for a package (PEP 440 version -> distribution) and get the metadata for
a distribution. A new trait `ResolverProvider` abstracts these two away and
allows replacing the real network requests e.g. with stored responses
(https://github.com/pradyunsg/pip-resolver-benchmarks/blob/main/scenarios/pyrax_198.json).
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Co-authored-by: konsti <konstin@mailbox.org>
