solver: consider virtual provides to exclude non-provides transitively
this fixes package selection when a 'real' package exists, but would need to be provided by another package with 'virtual provides'. In current package database this can happen with postgresql which is also provided by postgresql-bdr. Normally postgresql would be satisfied by postgresql, but if any package depends on postgresql-bdr and there's no versioned dependency on postgresql this will help apk figure out that postgresql-bdr should be used.
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