[libcalamares] Build complete instanceDescriptor list

- there's no reason to ignore custom instances that are **not**
  mentioned in the *instances* section: it may be useful to
  name more that one even without distinct config files.

src/libcalamares/Settings.cpp

@@ -268,28 +268,18 @@ Settings::validateSequence()
             if ( !k.isValid() )
             {
                 cWarning() << "Invalid instance key in *sequence*," << instance;
+                continue;
             }
-            if ( k.isValid() && k.isCustom() )
+
+            targetKey = k;
+            const auto it = std::find_if( m_moduleInstances.constBegin(), m_moduleInstances.constEnd(), moduleFinder );
+            if ( it == m_moduleInstances.constEnd() )
             {
-                targetKey = k;
-                const auto it
-                    = std::find_if( m_moduleInstances.constBegin(), m_moduleInstances.constEnd(), moduleFinder );
-                if ( it == m_moduleInstances.constEnd() )
+                if ( k.isCustom() )
                 {
                     cWarning() << "Custom instance key" << instance << "is not listed in the *instances*";
-                    // don't add it, let this fail later.
-                }
-            }
-            if ( k.isValid() && !k.isCustom() )
-            {
-                targetKey = k;
-                const auto it
-                    = std::find_if( m_moduleInstances.constBegin(), m_moduleInstances.constEnd(), moduleFinder );
-                if ( it == m_moduleInstances.constEnd() )
-                {
-                    // Non-custom instance, just mentioned in *sequence*
-                    m_moduleInstances.append( InstanceDescription( k ) );
                 }
+                m_moduleInstances.append( InstanceDescription( k ) );
             }
         }
     }

src/libcalamares/Settings.h

@@ -101,9 +101,23 @@ public:
     QStringList modulesSearchPaths() const;
 
     using InstanceDescriptionList = QList< InstanceDescription >;
+    /** @brief All the module instances used
+     *
+     * Each module-instance mentioned in `settings.conf` has an entry
+     * in the moduleInstances list -- both custom entries that are
+     * in the *instances* section, and each module mentioned in the
+     * *sequence*.
+     */
     InstanceDescriptionList moduleInstances() const;
 
     using ModuleSequence = QList< QPair< ModuleSystem::Action, QStringList > >;
+    /** @brief Representation of *sequence* of execution
+     *
+     * Each "section" of the *sequence* key in `settings.conf` gets an
+     * entry here, stating what kind of action is taken and which modules
+     * take part (in order). Each entry in the list is an instance key
+     * which can be found in the moduleInstances() list.
+     */
     ModuleSequence modulesSequence() const;
 
     QString brandingComponentName() const;