Fix cmake install targets (#9822)

* Avoid exporting build tree targets to installation directory

`export(TARGETS ...)` is meant to allow for build trees to be found
from other projects, however, writing the file to an installation
directory causes buildtree paths to be hardcoded in the installed
protobuf-targets.cmake

* Install protobuf-targets.cmake from target, not build tree

This allows for installation prefixes to match the intended
installation directories, instead of being copied from
the build tree.

cmake/install.cmake

@@ -128,24 +128,20 @@ configure_file(${protobuf_SOURCE_DIR}/cmake/protobuf-options.cmake
 if (protobuf_BUILD_PROTOC_BINARIES)
   export(TARGETS libprotobuf-lite libprotobuf libprotoc protoc
     NAMESPACE protobuf::
-    FILE ${CMAKE_INSTALL_CMAKEDIR}/protobuf-targets.cmake
+    FILE ${CMAKE_CURRENT_BINARY_DIR}/cmake/protobuf/protobuf-targets.cmake
   )
 else (protobuf_BUILD_PROTOC_BINARIES)
   export(TARGETS libprotobuf-lite libprotobuf
     NAMESPACE protobuf::
-    FILE ${CMAKE_INSTALL_CMAKEDIR}/protobuf-targets.cmake
+    FILE ${CMAKE_CURRENT_BINARY_DIR}/cmake/protobuf/protobuf-targets.cmake
   )
 endif (protobuf_BUILD_PROTOC_BINARIES)
 
 install(EXPORT protobuf-targets
+  FILE protobuf-targets.cmake
   DESTINATION "${CMAKE_INSTALL_CMAKEDIR}"
   NAMESPACE protobuf::
-  COMPONENT protobuf-export)
-
-install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_INSTALL_CMAKEDIR}/
-  DESTINATION "${CMAKE_INSTALL_CMAKEDIR}"
   COMPONENT protobuf-export
-  PATTERN protobuf-targets.cmake EXCLUDE
 )
 
 option(protobuf_INSTALL_EXAMPLES "Install the examples folder" OFF)