export: Allow multiple exports when only one export is installed

Since commit c8997fc046 (export: Allow depending on targets exported
multiple times, 2024-12-26, v4.0.0-rc1~241^2), it is possible to depend
on a target exported multiple times so long as the target is exported in
only one set and with a consistent namespace.  However, as a
side-effect, a target could not be in multiple export sets even if only
one of those sets was installed.

Update the check so that uninstalled export sets do not count towards a
target being exported multiple times.

Source/cmExportInstallFileGenerator.cxx

@@ -280,12 +280,14 @@ cmExportFileGenerator::ExportInfo cmExportInstallFileGenerator::FindExportInfo(
                     [&name](std::unique_ptr<cmTargetExport> const& te) {
                       return te->TargetName == name;
                     })) {
-      exportSets.insert(exp.first);
       std::vector<cmInstallExportGenerator const*> const* installs =
         exportSet.GetInstallations();
-      for (cmInstallExportGenerator const* install : *installs) {
-        exportFiles.push_back(install->GetDestinationFile());
-        namespaces.insert(install->GetNamespace());
+      if (!installs->empty()) {
+        exportSets.insert(exp.first);
+        for (cmInstallExportGenerator const* install : *installs) {
+          exportFiles.push_back(install->GetDestinationFile());
+          namespaces.insert(install->GetNamespace());
+        }
       }
     }
   }

Tests/RunCMake/PackageInfo/DependsMultipleNotInstalled.cmake

@@ -0,0 +1,4 @@
+project(DependsMultipleNotInstalled CXX)
+set(NAMESPACE foo::)
+include(DependsMultipleCommon.cmake)
+install(TARGETS foo EXPORT foo-alt) # set foo-alt never installed

Tests/RunCMake/PackageInfo/RunCMakeTest.cmake

@@ -34,3 +34,4 @@ run_cmake(LowerCaseFile)
 run_cmake(Requirements)
 run_cmake(TargetTypes)
 run_cmake(DependsMultiple)
+run_cmake(DependsMultipleNotInstalled)