Better handling of resource roots.

[Thom1729]

Previously, we handled the mapping between file and resource paths using get_resource_roots() and get_installed_resource_roots(). This PR consolidates the mapping logic into ResourceRoot objects, each representing a directory containing either zipped or unzipped packages. file_path() and from_file_path() merely iterate over the ResourceRoot objects and return the first result.

Other tweaks:

• Cache the list of roots instead of recomputing it each time.
• Instead of pathlib.Path.relative_to, use a custom implementation that doesn't spend most of its CPU time formatting an error message that we don't care about.
• Use a wrap_path function to avoid calling the (relatively expensive) Path constructor on Path objects.
• Avoid unnecessary reparsing when calling the ResourcePath constructor with a ResourcePath object as the first argument.

These changes improved the performance of a function that calls from_file_path() on all of the paths in sys.modules by more than a factor of three.

Currently, the ResourceRoot list isn't exposed or documented. This may be a useful primitive for packages that care where resources are located.