lib: support arbitrary compressed package files

We need to modify the matching of valid package files to support formats
like zstd. Let's try to use an eager approach instead of a simple
whitelist in order to be functional for arbitrary formats that may be
introduced in the future without the need to adjust any code.

Allow any single fragment word as compression type but filter out known
non-package content like signature files.

Signed-off-by: Levente Polyak <anthraxx@archlinux.org>

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