repo: refactor indexes to be more extensible

- virtual provider cache and tags were previously generated by nearly
  identical but separate methods.

- factor out an Indexer interface for updating repository caches, and
  provide implementations for each type of index (TagIndex,
  ProviderIndex) so that more can be added if needed.

- Among other things, this allows all indexes to be updated at once.
  This is an advantage because loading package files is the real
  overhead, and building the indexes once the packages are loaded is
  trivial. We avoid extra bulk read-ins by generating all package indexes
  at once.

- This can be extended for dependents (reverse dependencies) and patches
  later.

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