Make CMake package config backward compatible with FindCapnProto module

c++/cmake/CapnProtoConfig.cmake.in

+# Cap'n Proto CMake Package Configuration
+#
+# When configured and installed, this file enables client projects to find Cap'n Proto using
+# CMake's find_package() command. It adds imported targets in the CapnProto:: namespace, such as
+# CapnProto::kj, CapnProto::capnp, etc. (one target for each file in pkgconfig/*.pc.in), defines
+# the capnp_generate_cpp() function, and exposes some variables for compatibility with the original
+# FindCapnProto.cmake module.
+#
 # Example usage:
 #   find_package(CapnProto)
 #   capnp_generate_cpp(CAPNP_SRCS CAPNP_HDRS schema.capnp)
@@ -5,8 +13,26 @@
 #   add_executable(foo main.cpp ${CAPNP_SRCS})
 #   target_link_libraries(foo CapnProto::capnp)
 #
-#  If you are using RPC features, use 'CapnProto::capnp-rpc'
-#  in target_link_libraries call.
+# If you are using RPC features, use 'CapnProto::capnp-rpc' in the target_link_libraries() call.
+#
+# Paths to `capnp` and `capnpc-c++` are exposed in the following variables:
+#   CAPNP_EXECUTABLE
+#       Path to the `capnp` tool (can be set to override).
+#   CAPNPC_CXX_EXECUTABLE
+#       Path to the `capnpc-c++` tool (can be set to override).
+#
+# For FindCapnProto.cmake compatibility, the following variables are also provided. Please prefer
+# using the imported targets in new CMake code.
+#   CAPNP_INCLUDE_DIRS
+#       Include directories for the library's headers.
+#   CANP_LIBRARIES
+#       The Cap'n Proto library paths.
+#   CAPNP_LIBRARIES_LITE
+#       Paths to only the 'lite' libraries.
+#   CAPNP_DEFINITIONS
+#       Compiler definitions required for building with the library.
+#   CAPNP_FOUND
+#       Set if the libraries have been located (prefer using CapnProto_FOUND in new code).
 #
 @PACKAGE_INIT@
 
@@ -25,3 +51,23 @@ endif()
 
 include("${CMAKE_CURRENT_LIST_DIR}/CapnProtoTargets.cmake")
 include("${CMAKE_CURRENT_LIST_DIR}/CapnProtoMacros.cmake")
+
+
+# FindCapnProto.cmake provides dependency information via several CAPNP_-prefixed variables. New
+# code should not rely on these variables, but prefer linking directly to the imported targets we
+# now provide. However, we should still set these variables to ease the transition for projects
+# which currently depend on the find-module.
+
+set(CAPNP_INCLUDE_DIRS ${CAPNP_INCLUDE_DIRECTORY})
+
+# No need to list all libraries, just the leaves of the dependency tree.
+set(CAPNP_LIBRARIES_LITE CapnProto::capnp)
+set(CAPNP_LIBRARIES CapnProto::capnp-rpc CapnProto::capnp-json
+                    CapnProto::kj-http CapnProto::kj-test)
+
+set(CAPNP_DEFINITIONS)
+if(TARGET CapnProto::capnp AND NOT TARGET CapnProto::capnp-rpc)
+  set(CAPNP_DEFINITIONS -DCAPNP_LITE)
+endif()
+
+set(CAPNP_FOUND ${CapnProto_FOUND})