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.

Source/CPack/cmCPackGenerator.cxx

@@ -625,7 +625,8 @@ int cmCPackGenerator::InstallProjectViaInstallCMakeProjects(
         cmGlobalGenerator gg(&cm);
         cmsys::auto_ptr<cmMakefile> mf(
           new cmMakefile(&gg, cm.GetCurrentSnapshot()));
-        if (!installSubDirectory.empty() && installSubDirectory != "/") {
+        if (!installSubDirectory.empty() && installSubDirectory != "/" &&
+            installSubDirectory != ".") {
           tempInstallDirectory += installSubDirectory;
         }
         if (componentInstall) {