CMake: rename imported yamlcpp library

- call it yamlcpp::yamlcpp

This is source-incompatible if targets link explicitly to yamlcpp

CMakeModules/FindYAMLCPP.cmake

@@ -13,7 +13,7 @@
 #   YAMLCPP_LIBRARY, where to find yaml-cpp
 #   YAMLCPP_INCLUDE_DIR, where to find yaml.h
 # There is also one IMPORTED library target,
-#   yamlcpp
+#   yamlcpp::yamlcpp
 #
 # By default, the dynamic libraries of yaml-cpp will be found. To find the static ones instead,
 # you must set the YAMLCPP_STATIC_LIBRARY variable to TRUE before calling find_package(YamlCpp ...).
@@ -21,6 +21,10 @@
 # If yaml-cpp is not installed in a standard path, you can use the YAMLCPP_DIR CMake variable
 # to tell CMake where yaml-cpp is.
 
+if(TARGET yamlcpp::yamlcpp)
+    return()
+endif()
+
 # attempt to find static library first if this is set
 if(YAMLCPP_STATIC_LIBRARY)
     set(YAMLCPP_STATIC libyaml-cpp.a)
@@ -61,9 +65,9 @@ mark_as_advanced(YAMLCPP_INCLUDE_DIR YAMLCPP_LIBRARY)
 
 # Add an imported target
 if( YAMLCPP_LIBRARY )
-    add_library( yamlcpp UNKNOWN IMPORTED )
-    set_property( TARGET yamlcpp PROPERTY IMPORTED_LOCATION ${YAMLCPP_LIBRARY} )
+    add_library( yamlcpp::yamlcpp UNKNOWN IMPORTED )
+    set_property( TARGET yamlcpp::yamlcpp PROPERTY IMPORTED_LOCATION ${YAMLCPP_LIBRARY} )
     if ( YAMLCPP_INCLUDE_DIR )
-        set_property( TARGET yamlcpp PROPERTY INTERFACE_INCLUDE_DIRECTORIES ${YAMLCPP_INCLUDE_DIR} )
+        set_property( TARGET yamlcpp::yamlcpp PROPERTY INTERFACE_INCLUDE_DIRECTORIES ${YAMLCPP_INCLUDE_DIR} )
     endif()
 endif()