Run a check that macOS binaries have no external dependencies.

It's relatively easy to accidentally include a binary that is linked against something *not* in the app bundle - especially if Python dependencies are compiled.

Briefcase should perform a check as part of the build (or maybe the packaging) process that the built binary is independent of non-app resources.

The [delocate tool](https://github.com/matthew-brett/delocate) may be useful for this.