cxxmodules: generate synthetic targets as an initial pass

We need to be able to construct BMIs that will be usable from the client
modules for the target importing the module, so create BMI-only
compilation rules for `IMPORTED` targets to create these BMIs.

Source/cmFileSet.cxx

@@ -7,6 +7,7 @@
 #include <utility>
 #include <vector>
 
+#include <cmext/algorithm>
 #include <cmext/string_view>
 
 #include "cmsys/RegularExpression.hxx"
@@ -88,6 +89,12 @@ cmFileSet::cmFileSet(cmake& cmakeInstance, std::string name, std::string type,
 {
 }
 
+void cmFileSet::CopyEntries(cmFileSet const* fs)
+{
+  cm::append(this->DirectoryEntries, fs->DirectoryEntries);
+  cm::append(this->FileEntries, fs->FileEntries);
+}
+
 void cmFileSet::ClearDirectoryEntries()
 {
   this->DirectoryEntries.clear();