dnf: Implement DependsOn and RequiredBy

[Conan-Kudo]

This allows software centers to make decisions based on the
dependency tree.

Fixes: #106

[Conan-Kudo]

This PR currently doesn't actually have the logic implemented, mostly because I'm not sure how to do it yet. However, I figured creating this PR would make it easier to solicit some guidance on what I should be doing, or even if my starting approach is even right.

@dmach, @j-mracek, @jrohel, @hughsie: I would appreciate any help given here...

[hughsie]

make decisions based on the dependency tree

What's the 40,000ft use case? You have to be careful, and this is usually why SIMULATE gives you better results for what's going to happen.

[Conan-Kudo]

The main use-case for implementing this is that Plasma Discover 5.22 now uses this for informing the user of what will change on installation or removal of an application. In the future, it may be used to implement heuristics for determining when to trigger offline updates.

[hughsie]

for informing the user of what will change on installation or removal of an application

You need to use SIMULATE for this. There's no way of showing the correct results without knowing what you're going to do, i.e. what the rest of the transaction looks like.

[Conan-Kudo]

But that wouldn't also show what it depends on that's already installed, would it?

[hughsie]

No, it should show you the INSTALLING/REMOVING/UPDATING packages just like a normal "install" would do.

[Conan-Kudo]

Maybe I'm not explaining this correctly, so perhaps a picture shows this better:

This is from Plasma Discover on openSUSE with the Zypp backend. This currently doesn't work on Fedora or openSUSE with the DNF backend.

[hughsie]

Ahh yes; for this kind of thing it's fine -- my point was based on the assumption you wanted "if I installed this what else would have to be installed"

[Conan-Kudo]

I've been up for too many hours, I'm bad at words now. 😛

💤

[hughsie]

Okay, lets fix up the FIXMEs and then LGTM.