Schedule defered _userver_codegen_make_main_target only once

[ae-ignatiev]

Subsequent calls of cmake function _userver_codegen_register_files fails with error "userver-codegen target already defined".
Reproduces in case of adding grpc proto libs using userver_add_grpc_library in a subsequent call chain.

simplified example:

find_package(
    userver
    COMPONENTS grpc
    REQUIRED
)

userver_add_grpc_library(${PROJECT_NAME}-proto PROTOS samples/greeter.proto)

add_subdirectory(another_grpc_lib)
# there CMakeLists.txt inside another_grpc_lib with content like
# userver_add_grpc_library(another-lib-proto PROTOS samples/service.proto)
#

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[apolukhin]

We tried to reproduce the issue in 69f95f2 but the CMake keeps working fine

@ae-ignatiev please add a reproducer test to this PR