Enhance conflict error messages with the dependency chains that lead to the conflict

[crogoz]

When a conflict happens/or a package cannot be installed because one of the dependencies doesn't have any provider, libsolv would only show the specification of the package that cannot be installed.
It would be useful, however, to show what introduces the conflict and what is the chain of dependencies that lead to a conflict.

For example:
A user requests packages A (depends on package C 1.0.0) and B (depends on package C 2.0.0). This leads to a conflict which shows:
"Problem: package A requires C 1.0.0, but none of the providers can be installed"

It would be useful to also show that we requested 2 different versions of C and who required them (especially useful if the chain of dependencies up to the C library is very complicated A -> -> C 1.0.0, same with B).

[ignatenkobrain]

Not sure if this is something that should be done in the libsolv… I would imagine that this would be part of a package manager that uses libsolv.. Or some ext_utils could be in libsolv but everyone would like to have this information in slightly different form…

[crogoz]

Happy to move this issue in the package manager side, however I was thinking this may be more easily retrievable when printing the problem in libsolv, correct me if this is not true.
Do you know how we can get this information from libsolv if I am to move the logic to the package manager?
I'd like to show the conflicting versions of packages (C - 1.0.0 and C 2.0.0) and what package requires them.