Share in-flight map across resolutions (#932)
## Summary
This PR fixes a subtle bug in `pip install` when using `--reinstall`. If
a package depends on a build system directly (e.g., `waitress` depends
on `setuptools`), and then you have other packages that also need the
build system to build a source distribution, right now, we don't share
the `OnceMap` between those cases.
This lifts the `InFlight` tracking up a level, so that it's initialized
once per command, then shared everywhere.
## Test Plan
I'm having trouble coming up with an identical test-case and hesitant to
add this slow test to the suite... But if you run `pip install
--reinstall` with:
```
waitress @ git+https://github.com/zanieb/waitress
devpi-server @ git+https://github.com/zanieb/devpi#subdirectory=server
```
It fails consistently on `main` and passes here.
