Auto merge of #5782 - bundler:seg-global-gem-cache, r=indirect

[2.0] Add a global cache for downloaded .gem files

### What was the end-user problem that led to this PR?

The problem was that bundler would need to download `foo-1.0.gem` files from a RubyGems server for each different ruby version installed on a user's machine. It also meant that people installing into a per-app path would need to re-download every gem for that bundle an additional time. This adds up, and makes `bundle install` slower than it needs to be.

### Was was your diagnosis of the problem?

My diagnosis was that Bundler could keep a (per-source) cache of these `.gem` files, and pull from that cache instead of hitting the network whenever possible.

### What is your fix for the problem, implemented in this PR?

My fix implements said cache, in a very similar way to the compact index cache (same cache slug per remote strategy, etc). This largely comes from https://github.com/bundler/bundler/pull/3983.

### Why did you choose this fix out of the possible options?

I chose this fix because it is safe when used from multi-source gemfiles, it is easy to clear (`rm -rf bundle cache`), and it minimally interferes with the existing installation process.

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