rework cachesets API to allow future extension

The introduction of Fnmatch showed that each new selector would require
multiple new virtual methods in the CacheSetHelper to work correctly,
which isn't that great. We now flip to a single virtual method which
handles all cases separated by an enum – as new enum values can be added
without an ABI break.

Great care was taken to make old code work with the new way of organisation,
which means in return that you might be bombarded with deprecation
warnings now if you don't adapt, but code should still compile and work
as before as can be seen in apt itself with this commit.

Git-Dch: Ignore

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