CPack: Add compatibility for incorrect CPACK_INSTALL_CMAKE_PROJECTS value

Since commit v3.6.0-rc1~339^2 (CPack: Fix CPACK_INSTALL_CMAKE_PROJECTS
SubDirectory (4th) option, 2016-02-18) we now honor the "Directory"
option of this variable.  Prior to that commit the value was not used
so projects may have used a placeholder value like `.` instead of `/`.
Treat `.` the same as `/` to restore compatibility with such projects.

Fixes #16205.