Simplify pubkey header building code

[dmantipov]

At makePubkeyHeader(), the key has PGP data collected already,
so rpmPubkeyDig() is redundant. And, since the former is the only
user of the latter, which, in turn, mostly duplicates the
functionaliry of rpmPubkeyNew(), rpmPubkeyDig() may be dropped.

[pmatilai]

rpmPubkeyDig() is a public API function, we can't just drop it. At least current incarnations of libdnf actually even use it. Quite possibly for various wrong reasons, but still.

[DemiMarie]

We can’t drop it, but we can certainly stop using it internally :)

[dmantipov]

We can’t drop it, but we can certainly stop using it internally :)

OK let's reduce the change to internal scope.

[dmantipov]

rpmPubkeyDig() is a public API function, we can't just drop it.

How many (important) API dependencies we have? What if I do the corresponding patch for libdnf as well?

[pmatilai]

It's not a question of any individual user. Removing interfaces from public API requires a soname bump and those are not something to be done lightly because they cause pain to every single rpm (API) user. So it's not something you'd do for any one interface. What we can do is mark stuff deprecated and then remove those in a batch when there's enough of a reason for the soname bump.

[pmatilai]

Anyway, thanks for the patch!