cmake: add library aliases as find-package finds them

This allows to consumers to use the same targets regardless
whether capnproto CMakeList.txt is added as subdirectory
or whether capnproto is found as package

c++/src/capnp/CMakeLists.txt

@@ -56,6 +56,7 @@ set(capnp_schemas
   schema.capnp
 )
 add_library(capnp ${capnp_sources})
+add_library(CapnProto::capnp ALIAS capnp)
 target_link_libraries(capnp kj)
 #make sure external consumers don't need to manually set the include dirs
 target_include_directories(capnp INTERFACE
@@ -93,6 +94,7 @@ set(capnp-rpc_schemas
 )
 if(NOT CAPNP_LITE)
   add_library(capnp-rpc ${capnp-rpc_sources})
+  add_library(CapnProto::capnp-rpc ALIAS capnp-rpc)
   target_link_libraries(capnp-rpc capnp kj-async kj)
   install(TARGETS capnp-rpc ${INSTALL_TARGETS_DEFAULT_ARGS})
   install(FILES ${capnp-rpc_headers} ${capnp-rpc_schemas} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/capnp")
@@ -113,6 +115,7 @@ set(capnp-json_schemas
 )
 if(NOT CAPNP_LITE)
   add_library(capnp-json ${capnp-json_sources})
+  add_library(CapnProto::capnp-json ALIAS capnp-json)
   target_link_libraries(capnp-json capnp kj-async kj)
   install(TARGETS capnp-json ${INSTALL_TARGETS_DEFAULT_ARGS})
   install(FILES ${capnp-json_headers} ${capnp-json_schemas} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/capnp/compat")

c++/src/kj/CMakeLists.txt

@@ -57,6 +57,7 @@ set(kj-std_headers
   std/iostream.h
 )
 add_library(kj ${kj_sources})
+add_library(CapnProto::kj ALIAS kj)
 if(UNIX)
   target_link_libraries(kj PUBLIC pthread)
 endif()
@@ -82,6 +83,7 @@ set(kj-test-compat_headers
   compat/gtest.h
 )
 add_library(kj-test ${kj-test_sources})
+add_library(CapnProto::kj-test ALIAS kj-test)
 target_link_libraries(kj-test kj)
 install(TARGETS kj-test ${INSTALL_TARGETS_DEFAULT_ARGS})
 install(FILES ${kj-test_headers} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/kj")
@@ -103,6 +105,7 @@ set(kj-async_headers
 )
 if(NOT CAPNP_LITE)
   add_library(kj-async ${kj-async_sources})
+  add_library(CapnProto::kj-async ALIAS kj-async)
   target_link_libraries(kj-async kj)
   if(UNIX)
     # external clients of this library need to link to pthreads