Clean up files between bootstrap and build-sequence commands

[lcarva]

In build-wheels, fromager is called twice for each package being built. First, it calls the bootstrap command to generate the build-order.json file, then it calls build-sequence to what is listed in the build-order.json file.

In order to generate an accurate build-order.json file, fromager must build some of the dependencies. That's ok. The problem is that the content built by bootstrap is not really considered trusted by the fromager maintainers.

Let's change the build-wheels script to throw away any built content from the bootstrap command:

1. For each package
   1. Run bootstrap for the package.
   2. Delete all the generated files, except the build-order.json file.
2. For each build-order.json file
   1. Run build-sequence for the file.
   2. Clean up the work-dir. Keep all other files, including sdist-repo and wheel-repo. We want to re-use those when building multiple packages.