Generate proper Link metadata for resources in imported projects

Generate proper Link metadata for resources in imported projects

When importing a project that contains embedded resources, the id
of those resources must be generated using the path relative
to the project file that includes them, not relative to the
main project file. To fix this problem, we now generate a
Link metadata property for those resources that has the correct
relative path, and later this is used to generate the correct
id for the resource. We also now store a new metadata value
for all items (DefiningProjectFullPath) which contains the
path to the project file that defines the element. This is
used to check if an item was defined in an imported file,
in which case the Link assignment may need to be made.

This fixes bug BXC 20966.