ENH cmake: Target per Python command

[Lestropie]

On dev it's possible to build only one specific C++ binary (and all requisite dependencies to do so) by specifying --target at the command-line. This however can't be done for Python commands: everything is all done within a small number of targets: copying / linking API files, copying / linking source files, generating version file, building executable Python files.

It would be preferable to have one target per command, so that --target works as expected. Doing so would still need to compile all C++ binaries, copy / link all API files, and generate the version file, but would only copy / link source files and generate the executable Python file for that one command.

@daljit46 Is this something you could take on?

[daljit46]

Sure, happy to help. I think this is a good idea and will make our build code more consistent. BTW, what's your rationale for wanting this? Generally speaking, I only build individual C++ targets to reduce compilation time, but that wouldn't apply here since Python commands have no "compilation" step.

[Lestropie]

Mostly on principle. I have a couple of projects where I'm building multiple different versions of the software within a single container, and want full control over what I build within each. It makes more sense to me to have each individual commands show up in the build progress / logs. Also I'm getting into the routine of using cmake --target when doing development work on specific commands, and it makes sense if testing changes to a Python command to be able to do the same.

It'd be more effective if we had the ability to explicitly specify for each Python command which MRtrix C++ binaries are required, such that if a build nominates a specific Python command target, only those MRtrix C++ commands potentially invoked by that command need be built. Slightly trickier so deferring on that for now.