Improve performance of DefaultPolicy

[Toflar]

The $policy->selectPreferredPackages() is another bottleneck in the PoolOptimizer as we call it for every dependency group which can easily happen for 10 000+ times on medium size projects. Most packages match to a lot of dependency groups so we actually compare the same packages over and over again to sort them within one selectPreferredPackages() call. The respective 2 usort() calls were actually responsible for up to 25% of the whole PoolOptimizer::optimize() process.

With this PR we're now caching the result so the sorting can be way faster (another 0.5 - 1 seconds down on the PoolOptimizer).

Because we're using the PolicyInterface and Pool $pool is an argument, we have to make sure the cache is specific to the $pool instance. Thanks to the new PHP 7.2 requirement, we can use the faster spl_object_id() for that 😎

[Seldaek]

Have you tried caching the whole result like sort($nameLiterals); $cacheKey = implode(',', $nameLiterals); then memoize the whole usort() call instead of every individual comparison?

I am not entirely sure if the combinations of literals here will vary so much that it makes it useless or not.. But if not I guess it'd be even more efficient. Gotta try this with various projects though

[Toflar]

I think a complete match is pretty unlikely to happen. We compare within the dependency groups. So we have a group for e.g. ^4.2 and ^4.3 etc. Having the exact same packages inside those groups sounds very unrealistic to me. But I'll run some tests to see how many hits we'd have.

[Toflar]

Turns out I was wrong :) There are actually also thousands of hits 🥳 Both caches have very high hit rates now. I had to fix the cache key of the sorting cache too. I forgot to consider $requiredPackage.

[Seldaek]

Thanks