That restores support for compact index dummy implementations that only lists
versions, without checksums or dependencies.
This format is undocumented, so we may want to get rid of it in the
future. However, some of our tests rely on it, and some implementations
did use it (gems.mutant.dev at least). And the way the code was written
suggest that support was intentional.
So for now, we should restore it.
0427d8c983
This still allocates a ton (a string for each line, plus a bunch of
splits into arrays), but it helps a bit when Bundler has to go through
dependency resolution.
```
==> memprof.after.txt <==
Total allocated: 194.14 MB (2317172 objects)
Total retained: 60.81 MB (593164 objects)
==> memprof.before.txt <==
Total allocated: 211.97 MB (2404890 objects)
Total retained: 62.85 MB (640342 objects)
```
c68b41b0e5
