Adhoc signing happens too often

[mhsmith]

Adhoc signing of every .so or .dylib in the app takes at least 7 seconds on my machine for a minimal app, and 15 seconds for this app. So it's annoying that it happens every time I run briefcase run -u, and unnecessary because that command isn't capable of changing any of the signed files.

Similarly, briefcase package -u will adhoc sign the app, and then properly sign it all over again.

[freakboy3742]

Definitely agreed that some optimisation is called for here.

Some notes for potential implementors:

1. We can't completely drop signing on briefcase run -u - while we don't need to resign the app_packages, the signing of the app bundle as a whole is needed, because the app content has changed.
2. Build currently automatically does an automatic adhoc sign to ensure that run has a binary that can run locally. However, when invoking package, that signing during build is redundant. The good news is that the kwargs passed to build_app will have identity and adhoc_sign provided as arguments if it is being invoked transitively from a package command; this could be used as a signal to ignore the signing pass on build.
3. There is some crossover with Remove the --no-sign option and warn the user about --adhoc-sign on macOS applications #865; anyone tackling this issue may want to look into that one as well.
4. There is also potentially some crossover with Make create, update, build and run automatically execute any of the earlier steps which are out of date #807. Part of the underlying issue here is that the "dirty" state of the app includes whether the file has been updated since it was last signed. Updating a single binary file should only require that single file to be re-signed; the current process does a complete re-sign, regardless of whether a file has been altered. Addressing Make create, update, build and run automatically execute any of the earlier steps which are out of date #807 is a much bigger change; we can make significant improvements to workflows without introducing a full "dirty state" tracking system.