Uplift how artifacts are handled.

[stellaraccident]

Functional changes:

• Artifact archives now always write their manifest first (which for a tar file is important) and preserve the artifact directory structure (archives were being flattened as I hadn't fully thought through the case).
• The artifact-flatten command can now operate on exploded artifact directories or artifact archives.
• Target neutral artifacts are now named syntactically like their targetful counterparts but with a "_generic" suffix (e.g. "foo_dev_gfx100" vs "bar_dev_generic"). When working on Python packaging, I found myself regretting that these were syntactically different, as it made pattern matching harder.
• While some of this might be possible with just creative use of tar and friends, in my early experience having tried that, there are all kinds of corners with respect to attribute and symlink preservation when I tried to use other tools. I still think this tool is paying for itself because it is platform independent and doesn't have the preservation issues that were silently infecting some other attempts. It also works with trees of hardlinks, which is pretty important for performance of the build.

Additional cleanups:

• Removes THEROCK_ARTIFACT_ARCHIVE_TYPES: they are always .tar.xz now. This takes out some sketchy zip file handling too, which wasn't working out (zip is a very bad format for this and the numbers were terrible).
• Adds a unit test for fileset_tool.py. It doesn't have full coverage of everything yet, but tests the artifact->archive->flatten and round trip paths.
• Removes the configurable --manifest in favor of always generating in a fixed way.

[amd-chrissosa]

Those shorthands with pathlib are really cool!