Feature Request: Optimize `swift_debug_settings.py`

Similar to #1020.

[ ] In BwX mode, only set settings for top-level targets that have unfocused dependencies. Currently it will generate for all top-level targets if there are any unfocused targets anywhere in the build graph.
[x] Generate this file over in Starlark. Has the same requirement as moving link.params over there: FilePathResolver and related logic (mainly xcodeGeneratedFiles) needs to migrate over to Starlark. (https://github.com/buildbuddy-io/rules_xcodeproj/pull/1670)
[ ] Consider splitting the file per top-level target. The main file can load the file for the required top-level target. For some projects with lots of dependencies (and even more so if those dependencies are Objective-C) the settings for each top-level target is quite large. This should speed up module loading.
[ ] Generate the file(s) on-demand, as part of the xl/bl output groups

MobileNativeFoundation / rules_xcodeproj