feat: Allow files in wheels to be installed to directories

[Ahajha]

When specifying data_files in py_wheel, allow just the directory to be specified (with a trailing slash), in which case it will use the existing filename. This avoids duplicating (potentially platform-specific) names. Additionally, targets with multiple files can be installed as a group to a folder, with the same filename-preserving behavior.

Before, this would result in the file simply not being installed, so this only changes already-broken behavior.

PR Instructions/requirements

• Update CHANGELOG.md as applicable
• Update docs and tests, as applicable
• Delete these instructions prior to sending the PR

[Ahajha]

Sorry for the long pause while this sat here, been a busy few months but I got some time to come back to this. Still need to add a few tests + update docs, but the core functionality is ready.