Change allow_vendor_change default to false with descriptive hint

- When the solver blocks an update due to a vendor change restriction, print a hint suggesting
`--setopt=allow_vendor_change=true` (rpm-software-management#750)

A previous attempt to change this default was reverted because solver errors gave no indication
that vendor locking was the cause. The new hint in `print_resolve_hints()` addresses that by
detecting `RULE_UPDATE` problems and telling the user how to override the restriction.

Closes: rpm-software-management#750 SWMBZBUGSM-153 BZ#2219624

CI Tests: rpm-software-management/ci-dnf-stack#1839

Signed-off-by: Fellipe Henrique <me@fhbash.com>

Author: fhbash

dnf5/main.cpp

@@ -1107,6 +1107,7 @@ static void print_resolve_hints(dnf5::Context & context) {
         bool conflict = false;
         bool broken_file_dep = false;
         bool best = false;
+        bool vendor_change = false;
         // walk through all solver problem to detect a conflict, missing file dependency and best
         for (const auto & resolve_log : context.get_transaction()->get_resolve_logs()) {
             if (resolve_log.get_problem() == libdnf5::GoalProblem::SOLVER_ERROR) {
@@ -1129,6 +1130,9 @@ static void print_resolve_hints(dnf5::Context & context) {
                             case libdnf5::ProblemRules::RULE_BEST_2:
                                 best = true;
                                 break;
+                            case libdnf5::ProblemRules::RULE_UPDATE:
+                                vendor_change = true;
+                                break;
                             default:
                                 break;
                         }
@@ -1151,6 +1155,11 @@ static void print_resolve_hints(dnf5::Context & context) {
             }
         }
 
+        if (!conf.get_allow_vendor_change_option().get_value() && vendor_change) {
+            const std::string_view arg{"--setopt=allow_vendor_change=true"};
+            hints.emplace_back(libdnf5::utils::sformat(_("{} to allow changing package vendors"), arg));
+        }
+
         if (broken_file_dep) {
             const std::string_view arg{"--setopt=optional_metadata_types=filelists"};
             auto optional_metadata = conf.get_optional_metadata_types_option().get_value();