Reduce memory footprint by inlining the kind parameter

[Slack post](https://bazelbuild.slack.com/archives/D04TBHUQF53/p1759259799565349) Note from @fmeum: > Hey, just happened to look into https://github.com/cgrindel/rules_swift_package_manager/blob/main/swiftpkg%2Finternal%2Fmodule_maps.bzl. You can save memory by inlining the kind parameter. The format call will result in a unique new string per call to _add_headers when there are really only four or so string constants