Miscellaneous build system improvements

[Twisol]

• Tickets addressed: N/A
• Review: By commit
• Merge strategy: Merge (no squash)

Description

This PR attempts to simplify the current CMake-based build process, as well as improve its reliability in invalidating all appropriate targets when rebuilding. The changes include:

• Removing unnecessary NEW policy settings when cmake_minimum_required() already sets them to NEW;
• Removing reliance on the sole remaining OLD policy by switching to the modern FindPython3 method of bringing the Python language components into scope;
• Enabling automatically dependency-tracking through SWIG %includes, in the same way that C files already enjoy automatic tracking of #include dependencies;
• Preventing build dirty-looping due to custom commands not properly declaring their outputs. (Dirty-looping is when, if you run two builds in succession with no intervening source changes, the latter build thinks it still has some work to do.)

along with some other formatting and style cleanups.

Verification

By CI.

Documentation

No documentation is invalidated by this change, as far as I am aware.

Future work

• Our current collection of CMakeLists.txt could still use a lot of cleanup and reorganization.
• The conan build command appears to check and re-check dependencies several times for no apparent reason.
• The SWIG message payload interface generation step incurs a noticeable about of additional complexity, both in the build system (the .i.in wasn't considered a "dependency" before this PR) and at runtime (the generated .is often contain redundant code only necessary for one of them; but we have no way to add code to just one of them). Manually managing smaller, more tractable .i files for each message payload type would give us more clarity into the build and more control over what goes into each .i, while incurring a minimum of extra overhead (new messages also need their own new .i, but this can generally be copied from an adjacent message).

[patkenneally]

This is great. We're starting down the path to a sensible code base. Very exciting.