apt_info.py: fix apt_upgrades_pending and apt_upgrades_held (#207)

* apt_info.py: fix apt_upgrades_pending and apt_upgrades_held
* No need to test if package is installed for upgradable packages

Signed-off-by: kennethso168 <kennethso168@gmail.com>

apt_info.py

@@ -52,12 +52,8 @@ def _convert_candidates_to_upgrade_infos(candidates):
 
 
 def _write_pending_upgrades(registry, cache):
-    # Discount any changes that apply to packages that aren't installed (e.g.
-    # count an upgrade to package A that adds a new dependency on package B as
-    # only one upgrade, not two). See the following issue for more details:
-    # https://github.com/prometheus-community/node-exporter-textfile-collector-scripts/issues/85
     candidates = {
-        p.candidate for p in cache.get_changes() if p.is_installed and p.marked_upgrade
+        p.candidate for p in cache if p.is_upgradable
     }
     upgrade_list = _convert_candidates_to_upgrade_infos(candidates)
 
@@ -69,7 +65,10 @@ def _write_pending_upgrades(registry, cache):
 
 
 def _write_held_upgrades(registry, cache):
-    held_candidates = {p.candidate for p in cache if p.is_upgradable and p.marked_keep}
+    held_candidates = {
+        p.candidate for p in cache
+        if p.is_upgradable and p._pkg.selected_state == apt_pkg.SELSTATE_HOLD
+    }
     upgrade_list = _convert_candidates_to_upgrade_infos(held_candidates)
 
     if upgrade_list: