Refactor: replace our ad-hoc path handling with pathlib

[igalic]

We have a lot of ad-hoc path-handling that is probably better handled by a library.

Enter, pathlib

In particular validation:

• Theoretical: is this string a valid path?
• and practical: Is this path a Symlink?

Paths are dangerous! (#111) and should be handled with care.

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[igalic]

one thing we discovered while auditing the code was that we have to shield against path injection attacks by always using the correct, fully qualified path

[gronke]

Discussing this further with @igalic, here are our action items:

• create iocage datatype that inherits from pathlib that allows specifying paths relative to a resource
• str() returns the full absolute path after a safety check that the realpath is relative to the linked resource

[igalic]

additionally, for well known executables, we'd want to ensure they aren't symlinks.