de-duplicate version strings in the cache

Turns out that version numbers aren't as random as you might guess.
In my cache for example, I have:
  Total package names: 69513 (1390 k)
  Total package structures: 188259 (9036 k)
  Total distinct versions: 186345 (13.4 M)
  Total dependencies: 2052242 (57.5 M)
which amounts to 1035873 (10,1 M) strings.
Reusing version strings reduces this to 161465 (3.479 k).

This comes at a cost of course: Generation is slightly slower, but we
are still faster than what we started with and it makes room (also cache
size wise) for further changes.

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