loader.py: Separate functions for each operating system

[cclauss]

Separate functions for each operating system.

+ def _lib_candidates_linux():
+     """Yield possible libmagic library names on Linux."""
+
+ def _lib_candidates_macos():
+     """Yield possible libmagic library names on macOS."""
+
+ def _lib_candidates_windows():
+     """Yield possible libmagic library names on Windows."""

Related to:

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[ahupp]

Seems reasonable, thanks!