Refresh BuildDispatch when running pip install with --reinstall (#933)

## Summary

This fixes an extremely subtle bug in `pip install --reinstall`, whereby
if you depend on `setuptools` at the top level, we end up uninstalling
it after resolving, which breaks some cached state. If we have
`--reinstall`, we need to reset that cached state between resolving and
installing.

## Test Plan

Running `pip install --reinstall` with:

```txt
setuptools
devpi @ e334eb4dc9/devpi-2.2.0.tar.gz
```

Fails on `main`, but passes.

scripts/editable-installs/black_editable/pyproject.toml

@@ -13,6 +13,5 @@ license = {text = "MIT"}
 requires = ["pdm-backend"]
 build-backend = "pdm.backend"
 
-
 [tool.pdm]
 package-type = "library"