Fix node handling on package addition/removal

* UnpackingLeafNode: Add a fFinalPackageNode attribute. It is set when
the node is about to be removed and will point to the node's previous
head package node and be used in its stead. From the perspective of
the FS hooks this leaves the node in an unchanged state.
* Unpacking[Leaf,Directory]Node:
- Add WillBeFirstPackageNode(), returning whether the given package
node would become the head package node when added.
- Add PrepareForRemoval() which removes all package nodes. In case of
UnpackingLeafNode it also sets fFinalPackageNode.
- Add CloneTransferPackageNodes(). It is only implemented for
UnpackingLeafNode. It clones the node, transfers all package nodes
to the clone and sets fFinalPackageNode on this node.
* Volume::_{Add,Remove}PackageNode(): Solved the following TODO: When a
package is added or removed and a file present in both the
added/removed package and another package with the version in the
former having precedence, we have to remove the node (leaving it
unchanged) and replace it by a new node. This prevents clients having
the node opened or mapped from suddenly seeing different data. It also
fixes unbalanced calls to PackageNode::VFSInit()/VFSUninit() which
would result in file descriptors to package files being leaked.

Add UnpackingNode::IsOnlyPackageNode()

It returns whether the given package node is the only package node
attached to that node.

Remove package related dependencies from Node

* Move package related methods from Node to new interface UnpackingNode.
* LeafNode and Directory derive from UnpackingNode now.
* Adjust Volume implementation accordingly.

Fix node handling on package addition/removal

* UnpackingLeafNode: Add a fFinalPackageNode attribute. It is set when
the node is about to be removed and will point to the node's previous
head package node and be used in its stead. From the perspective of
the FS hooks this leaves the node in an unchanged state.
* Unpacking[Leaf,Directory]Node:
- Add WillBeFirstPackageNode(), returning whether the given package
node would become the head package node when added.
- Add PrepareForRemoval() which removes all package nodes. In case of
UnpackingLeafNode it also sets fFinalPackageNode.
- Add CloneTransferPackageNodes(). It is only implemented for
UnpackingLeafNode. It clones the node, transfers all package nodes
to the clone and sets fFinalPackageNode on this node.
* Volume::_{Add,Remove}PackageNode(): Solved the following TODO: When a
package is added or removed and a file present in both the
added/removed package and another package with the version in the
former having precedence, we have to remove the node (leaving it
unchanged) and replace it by a new node. This prevents clients having
the node opened or mapped from suddenly seeing different data. It also
fixes unbalanced calls to PackageNode::VFSInit()/VFSUninit() which
would result in file descriptors to package files being leaked.

Add UnpackingNode::IsOnlyPackageNode()

It returns whether the given package node is the only package node
attached to that node.

Remove package related dependencies from Node

* Move package related methods from Node to new interface UnpackingNode.
* LeafNode and Directory derive from UnpackingNode now.
* Adjust Volume implementation accordingly.