Best-effort to assert whether a version refers is a git sha digest or a tag. When it is, we
avoid re-downloading it if it's already fetched. But when it isn't, and thus refer to a branch,
we always re-download it. Note however that the download might be short-circuited by the
system-wide package cache, so a download doesn't actually mean a network request.
The package cache is however smart-enough to assert whether a package in the cache must be
re-downloaded (using HTTP ETag). So this is mostly about delegating the re-downloading logic to
the global packages cache.
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