Merge resolve and resolve_multi

[jsirois]

These functions are both part of the informal public API. We should deprecate passing a single interpreter and a single platform and just support passing optional lists for each as done in resolve_multi. We should also probably pick the name 'resolve' for the merged function, essentially deleting the current resolve and replacing it with the current resolve_multi renamed to resolve.

Resolving this issue is probably a good time to take up #584 and set up proper deprecation, then removal.

1. Create a deprecation policy doc.
2. Implement a deprecation system - emit pex_warning messages as needed.
3. Change resolve to support None, singles, or lists for interpreter and platform and deprecate both calling resolve with singles and calling resolve_multi.
4. When the deprecation cycle is up remove resolve and rename resolve_multi to resolve.

This deprecation should probably use version 3.0.0 as the boundary to conform to Semver.

[jsirois]

Additionally it would be good to circle back to the exposed datatypes and exception types and thoroughly document them and check their args as part of this resolve API cleanup and the API formalization umbrelled by #584.

[jsirois]

Since Pex has moved away from suporting any API besides the CLI, this can now be done without all the above prep work needed to support breaking CLI API changes.