Store source distribution builds under a unique manifest ID (#1051)
## Summary
This is a refactor of the source distribution cache that again aims to
make the cache purely additive. Instead of deleting all built wheels
when the cache gets invalidated (e.g., because the source distribution
changed on PyPI or something), we now treat each invalidation as its own
cache directory. The manifest inside of the source distribution
directory now becomes a pointer to the "latest" version of the source
distribution cache.
Here's a visual example:
With this change, we avoid deleting built distributions that might be
relied on elsewhere and maintain our invariant that the cache is purely
additive. The cost is that we now preserve stale wheels, but we should
add a garbage collection mechanism to deal with that.
