Add --archive option to use an already-extracted Python build #92

xvenv/xbuild/xpython all accept --platform/--arch/--cache to download (and cache) a target-platform Python build. There's no way to point at an already-extracted build on disk and skip the network/cache entirely.

Add a new --archive PATH option (mutually exclusive with --cache) that uses the given directory as if it were the already-extracted archive that --platform + --cache would have produced.