find_package: Add experimental gate for CPS support

Source/cmExperimental.cxx

@@ -46,6 +46,16 @@ cmExperimental::FeatureData LookupTable[] = {
     {},
     cmExperimental::TryCompileCondition::Always,
     false },
+  // ImportPackageInfo
+  { "ImportPackageInfo",
+    "e82e467b-f997-4464-8ace-b00808fff261",
+    "CMAKE_EXPERIMENTAL_FIND_CPS_PACKAGES",
+    "CMake's support for importing package information in the Common Package "
+    "Specification format (via find_package) is experimental. It is meant "
+    "only for experimentation and feedback to CMake developers.",
+    {},
+    cmExperimental::TryCompileCondition::Always,
+    false },
   // ExportPackageInfo
   { "ExportPackageInfo",
     "b80be207-778e-46ba-8080-b23bba22639e",

Source/cmExperimental.h

@@ -20,6 +20,7 @@ public:
     ExportPackageDependencies,
     WindowsKernelModeDriver,
     CxxImportStd,
+    ImportPackageInfo,
     ExportPackageInfo,
     ExportBuildDatabase,
 

Source/cmFindPackageCommand.cxx

@@ -23,6 +23,7 @@
 
 #include "cmAlgorithms.h"
 #include "cmDependencyProvider.h"
+#include "cmExperimental.h"
 #include "cmList.h"
 #include "cmListFileCache.h"
 #include "cmMakefile.h"
@@ -829,7 +830,9 @@ bool cmFindPackageCommand::InitialPass(std::vector<std::string> const& args)
   // Check and eliminate search modes not allowed by the args provided
   this->UseFindModules = configArgs.empty();
   this->UseConfigFiles = moduleArgs.empty();
-  if (this->UseConfigFiles && true /* FIXME check experimental flag */) {
+  if (this->UseConfigFiles &&
+      cmExperimental::HasSupportEnabled(
+        *this->Makefile, cmExperimental::Feature::ImportPackageInfo)) {
     this->UseCpsFiles = this->Configs.empty();
   } else {
     this->UseCpsFiles = false;

Tests/FindPackageCpsTest/CMakeLists.txt

@@ -1,6 +1,8 @@
 cmake_minimum_required(VERSION 3.31)
 project(FindPackageCpsTest)
 
+set(CMAKE_EXPERIMENTAL_FIND_CPS_PACKAGES "e82e467b-f997-4464-8ace-b00808fff261")
+
 # Protect tests from running inside the default install prefix.
 set(CMAKE_INSTALL_PREFIX "${CMAKE_CURRENT_BINARY_DIR}/NotDefaultPrefix")