License injection has issues

As articulated in pypa/setuptools#4981, there are some outstanding problems with the setuptools-based license injection.

- [ ] Injecting the license as a file leaves the file on the file system for the user to wonder why it's there. At the very least, this file should be omitted for local builds. Better would be to have a cleaner way to inject the license during sdist generation.
- [x] The license injection, including the network access, runs when building sdist to wheel, which is redundant and unnecessary. It's a larger problem for setuptools (there are many things that should probably be separated between sdist and wheel builds, but that's a separate issue). At the very least, the license retrieval should happen once for any source tree.

Start with the problems and proposed context in pypa/setuptools#4981, then trace how this skeleton injects licenses during sdist and wheel builds. Confirm whether retrieval and file creation occur in both paths; done means avoiding redundant retrieval and preventing unwanted license files in local builds.

Written by the indexing model from the issue text.