Use _cpython_tags, _generic_tags, and _compatible_tags
Use _cpython_tags, _generic_tags, and _compatible_tags

Since these functions are copies of the existing code, there is no
behavior change except each tag will now be present 3 times. To
accommodate this we remove all but the first duplicate tag from the
set of all tags.

We put `_compatible_tags` last because it will provide the lowest
priority tags. The order of `_cpython_tags` and `_generic_tags`
here is not significant - when we start customizing them we will
introduce a condition so that they are mutually exclusive.

pradyunsg/pip,xavfernandez/pip,sbidoul/pip,xavfernandez/pip,sbidoul/pip,pypa/pip,pfmoore/pip,pradyunsg/pip,xavfernandez/pip,pfmoore/pip,pypa/pip