Share a single `Index` across resolutions (#906)
## Summary
This PR uses a single `Index` that's shared between the top-level
resolver and any sub-resolutions happen in the course of that top-level
resolution (namely, to resolve build dependencies for any source
distributions).
In theory it's an optimization, since (e.g.) if we have two packages
that both need the `flit-core` build system, and we attempt to build
them both at once, we'll only fetch its metadata _once_, and share it
across the two resolutions. In practice, I haven't been able to get this
to show up in benchmarks. I suspect you'd need a _lot_ of source
distributions for it to matter... Though it may still be worth doing, it
strikes me as a cleaner design.
Closes #200.
Closes #541.
