Do some validation of packages in the system cache

[munificent]

Previously, there was an issue where pub could end up with an empty directory in the system cache. That would lead it to think the package had been successfully cached but was empty, which caused those "doesn't have a pubspec.yaml" errors people were/are seeing.

The root cause of that bug has been fixed, but:

1. Some people may still have existing broken system caches from before this fix.
2. There may be other unknown ways that a package could broken in your cache.

Given that, it might be good to do some validation of packages in the cache at pub install/update time. If the package isn't valid, we'll blow it away and install it again. Trivial validation would be if the directory is empty, it's bad, but we could possibly do more than that.

I still think prevention is better than a cure, and we should be in better shape for that now that pub downloads packages to a temp directory, but this is worth keeping in mind if we see other issues here.

[dgrove]

What about computing an MD5 over the directory, and recording that as part of the pub upload process?

[munificent]

Right. That's one of the things we discussed. The challenge I see with that is that then validating that MD5 hash means walking the entire directory, which might be too costly to do every time you pub install.

[munificent]

Removed this from the Later milestone.
Added this to the M3 milestone.

[munificent]

Set owner to @munificent.
Added Started label.

[munificent]

Added Accepted label.

[munificent]

Added Started label.

[munificent]

pub install will now re-install a previously cached package if it sees that the cached directory is missing a pubspec or has an empty lib directory.

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Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/pub#206.