Environment views: dependents before dependencies, resolve identical file conflicts (#42350)

Fix two separate problems:

1. We want to always visit parents before children while creating views
   (when it comes to ignoring conflicts, the first instance generated in
   the view is chosen, and we want the parent instance to have precedence).
   Our preorder traversal does not guarantee that, but our topological-
   order traversal does.
2. For copy style views with packages x depending on y, where
   <x-prefix>/foo is a symlink to <y-prefix>/foo, we want to guarantee
   that:
   * A conflict is not registered
   * <y-prefix>/foo is chosen (otherwise, the "foo" symlink would become
     self-referential if relocated relative to the view root)

   Note that
   * This is an exception to [1] (in this case the dependency instance
     overrides the dependent)
   * Prior to this change, if "foo" was ignored as a conflict, it was
     possible to create this self-referential symlink

Add tests for each of these cases

0 files changed, 0 insertions, 0 deletions