Reduce memory footprint by inlining the kind parameter

[cgrindel]

Slack post

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