cmState: Track known projects

Add a member to directory state that records the set of known projects.
Add a method to query whether a project exists.

Internally, while this doesn't allow us to confirm that a variable like
`foo_VERSION` is actually associated with a project `foo`, it provides a
much stronger indicator than the mere existence of said variable.

Source/cmStatePrivate.h

@@ -5,6 +5,7 @@
 
 #include "cmConfigure.h" // IWYU pragma: keep
 
+#include <set>
 #include <string>
 #include <unordered_map>
 #include <vector>
@@ -82,6 +83,8 @@ struct cmStateDetail::BuildsystemDirectoryStateType
   std::vector<std::string> NormalTargetNames;
   std::vector<std::string> ImportedTargetNames;
 
+  std::set<std::string> Projects;
+
   std::string ProjectName;
 
   cmPropertyMap Properties;

Source/cmStateSnapshot.cxx

@@ -5,10 +5,12 @@
 
 #include <algorithm>
 #include <cassert>
+#include <set>
 #include <string>
 #include <unordered_map>
 
 #include <cm/iterator>
+#include <cmext/algorithm>
 
 #include "cmDefinitions.h"
 #include "cmLinkedTree.h"
@@ -413,6 +415,7 @@ cmStateDirectory cmStateSnapshot::GetDirectory() const
 void cmStateSnapshot::SetProjectName(std::string const& name)
 {
   this->Position->BuildSystemDirectory->ProjectName = name;
+  this->Position->BuildSystemDirectory->Projects.insert(name);
 }
 
 std::string cmStateSnapshot::GetProjectName() const
@@ -420,6 +423,11 @@ std::string cmStateSnapshot::GetProjectName() const
   return this->Position->BuildSystemDirectory->ProjectName;
 }
 
+bool cmStateSnapshot::CheckProjectName(std::string const& name) const
+{
+  return cm::contains(this->Position->BuildSystemDirectory->Projects, name);
+}
+
 cmPackageState& cmStateSnapshot::GetPackageState(
   std::string const& packagePath)
 {

Source/cmStateSnapshot.h

@@ -57,6 +57,7 @@ public:
 
   void SetProjectName(std::string const& name);
   std::string GetProjectName() const;
+  bool CheckProjectName(std::string const& name) const;
 
   cmPackageState& GetPackageState(std::string const& packagePath);