I think we have to distinguish two features here:
- running custom code as part of the build process. We have a draft proposal for this here: Package Manager Extensible Build Tools
- specifying tasks which can be run manually outside of the build process, this is what @orta is proposing here.
These might seem similar, but I think we'd intentionally want to separate them in SwiftPM. We want to be much more strict about what happens during the build process due to the reasons outlined in the extensible build tools draft proposal, but tasks which are run manually do not need to meet the same high bar.