Allow package lookups across multiple indexes via explicit opt-in #2815
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Summary
This partially revives #2135 (with some modifications) to enable users to opt-in to looking for packages across multiple indexes.
The behavior is such that, in version selection, we take any compatible version from a "higher-priority" index over the compatible versions of a "lower-priority" index, even if that means we might accept an "older" version.
Closes #2775.