Fix dependency cycle between backend/native/subsystems and backend/python/subsystems

[Eric-Arellano]

Problem

backend/python/subsystems/python_native_code.py was depending on pants.backend.native.subsystems, but not declaring the dependency in its BUILD file.

The naive solution of adding the proper depedency to its BUILD results in a dependency cycle, as backend/native/subsystems/conan.py already depends depending on pants.backend.python.subsystems.python_tool_base.

So, ./pants test tests/python/pants_test/backend/python/tasks:pytest_run would fail when ran directly, but pass in CI because all of the targets would get the sources combined.

When changing to using the V2 test runner, we no longer allow this sort of leaky dependency usage—every dependency must be properly declared in the appropriate BUILD file. So, ./pants test tests/python/pants_test/backend/python/tasks:pytest_run started failing in #7724.

Solution

Move conan.py into a new subdirectory backend/native/subsystems/packaging, as suggested by @cosmicexplorer.