Improve path resolution

[bug-or-feature]

Fix for #1603. This PR:

• refactors path resolution functions in syscore.fileutils to delegate OS specific functionality to pathlib
• resolves absolute paths using a single variable at module load time using importlib.util.find_spec
• replaces file path manipulation code using os with pathlib
• simplifies and refactors resolution of private config files
• adds main functions for sysproduction scripts where needed
• adds extensive unit tests for syscore.fileutils, and runs them in CI with all OS flavours
• adds more unit tests for sysdata.config.configdata

Any questions, issues or concerns please shout