Yes, Previews are for working with Xcode. but I'm using Xcode to develop the package. I made a separate file because I didn't want to dump 100+ lines for a Preview into the source.
if you are using xcode, there is a preview content that is stripped out in release build but i haven't tried that with individual packages
for packages what i usually do is define a DebugXYZ target (for each corresponding target) and in xcode configuration set any target to be stripped that contains DEBUG* in the EXCLUDED_SOURCE_FILE_NAMES or EXCLUDED_EXPLICIT_TARGET_DEPENDENCIES (i'm pretty sure that is what preview content is doing behind the scenes)
from apple
and a helpful explainer
one additional option
there is one other way which is to use if DEBUG inline with your previews which will only be active when using xcode (it implicitly defines that for directly included/depended on packages) bit its harder to exclude resources afaict so not so great maybe...