Normalize wheel RECORD paths (on Windows)

[texodus]

#2870 refactored module_writer.rs, and in doing so removed a line which reformatted PathBuf rendered platform-specific Strings to exclusively us /:

// The zip standard mandates using unix style paths
let target = target.to_str().unwrap().replace('\\', "/");

While it seems this is technically allowed, some tools choke on this. In my case, the wheel package does not like windows-style paths in the RECORD file (even on Windows itself).

I've also added a unit test to validate this - though because the implementation uses PathBuf::to_string_lossy, it will only actually fail on Windows.

[messense]

Thanks for the PR, Windows CI is failing, we need to get it fixed to merge this.