feat: explicit symlink indicator

[lczyk]

a quick braindump / feature proposal. here is what we end up often doing in chisel-releases:

slices:
  bins:
    contents:
      /usr/sbin/ip6tables-nft-restore:  # Symlink to xtables-legacy-multi
      /usr/sbin/ip6tables-nft-save:  # Symlink to xtables-legacy-multi
      /usr/sbin/ip6tables-restore-translate:  # Symlink to xtables-legacy-multi
      ...

this is to make it easier to maintainers to unserstand which files ought to go together and how they relate to one another.

we could create the symlinks explicitly:

slices:
  bins:
    contents:
      /usr/sbin/ip6tables-nft-restore:
        symlink: xtables-legacy-multi
      /usr/sbin/ip6tables-nft-save:
        symlink: xtables-legacy-multi
      /usr/sbin/ip6tables-restore-translate:
        symlink: xtables-legacy-multi
      ...

but them we're not actually checking that that file really is a symlink. we're just making one. the problem is with release-to-release maintenance:

1. the underlying file e.g. /usr/sbin/ip6tables-nft-restore could be removed, and we would still be creating a symlink
2. the symlink target in the deb could change, and we would overwrite it
3. the file could change from symlink to binary and we would not catch it.

---

the very rough idea would be something like:

slices:
  bins:
    contents:
      /usr/sbin/ip6tables-nft-restore:
        expect-symlink: xtables-legacy-multi

which is a way of saying: "include this file. i expect it to be a symlink to $0. error otherwise"

i'm not strongly attached to the idea, but just thought i'd drop it here as a food-for-thought. :)