Multiple libraries in a single buildsystem can be exported to multiple
export-sets.

If a library from one export set depends on a library from
another export set, the export logic generates a check in the targets
file to verify that the target in the other export set is found. That
check is executed at downstream-find_package-time.

However, a target may depend privately on a target in another export
set. In that case, the depend used to also be listed in the required
targets in the targets file. Change the export logic to ignore the
private link entries instead.