Package Manager should ignore default.settings.php and default.services.yml

Problem/Motivation

Right now, Automatic Updates tries to allow default.settings.php and default.services.yml to be updated. This causes a lot of trouble because these files are in sites/default, which is normally write-protected by Drupal (and, indeed, it happens every time one visits the status report page; system_requirements() does the write protecting).

This means that in some situations, updates will fail because those two files cannot be copied into the write-protected directory.

Proposed resolution

But why, oh why, do we even care about updating these two files? They're only used once, by Drupal's installer, and then not used again by any existing site.

Given that they cause so much trouble, we should probably just ignore them completely.

Remaining tasks

Exclude sites/default/default.settings.php and sites/default/default.services.yml from all Package Manager operations.

Since those files generally get scaffolded into sites/default when Drupal core is updated, we should also implement a PostCreateEvent listener which makes sites/default writable in the stage directory.