[frontend] Refactor Package#last_build_reason

We've been temporarely converting the xml string, eg. '<a></b>' to
a Nokogiri::XML object before creating a hash from it.
By using Xmlhash#parse we can skip the intermediate step and directly
create a hash.
This reduces the code and slightly improves the performance of that
method.

src/api/app/controllers/webui/packages/build_reason_controller.rb

@@ -8,7 +8,7 @@ class BuildReasonController < WebuiController
 
       def index
         @details = @package.last_build_reason(@repository, @architecture.name, @package_name)
-        return if @details.explain
+        return if @details.explain.present?
 
         redirect_back(fallback_location: package_binaries_path(package: @package, project: @project, repository: @repository.name),
                       notice: "No build reason found for #{@repository.name}:#{@architecture.name}")

src/api/app/models/package.rb

@@ -1442,15 +1442,14 @@ def self.what_depends_on(project, package, repository, architecture)
   def last_build_reason(repo, arch, package_name = nil)
     repo = repo.name if repo.is_a? Repository
 
-    xml_data = Nokogiri::XML(BuildReasonFile.new(
+    xml_data = BuildReasonFile.new(
       project_name: project.name,
       package_name: package_name || name,
       repo: repo,
       arch: arch
-    ).to_s).xpath('reason')
-
-    data = Hash.from_xml(xml_data.to_s)['reason']
+    )
 
+    data = Xmlhash.parse(xml_data.to_s)
     # ensure that if 'packagechange' exists, it is an Array and not a Hash
     # Bugreport: https://github.com/openSUSE/open-build-service/issues/3230
     data['packagechange'] = [data['packagechange']] if data && data['packagechange'].is_a?(Hash)