Port squashfs lib to use github.com/diskfs/go-diskfs

[wagoodman]

Today we are using a fork of https://github.com/CalebQ42/squashfs to fulfill sif functionality. We should consider switching to github.com/diskfs/go-diskfs/filesystem/squashfs , the same as used in syft for squashfs processing (introduced in https://github.com/anchore/syft). This would mean that we can share the same optimized reader in syft file.Resolvers without making any changes.