cmake: Teach -E copy_directory to support multiple input directories

Help/manual/cmake.1.rst

Help/release/dev/cmake-E-copy-multiple-inputs.rst

Source/cmcmd.cxx

...rectory-three-source-files-target-is-directory-result.txt

...rectory-three-source-files-target-is-directory-stderr.txt

...py_directory-three-source-files-target-is-file-result.txt

...py_directory-three-source-files-target-is-file-stderr.txt

...rectory-three-source-files-target-is-not-exist-result.txt

...rectory-three-source-files-target-is-not-exist-stderr.txt

Tests/RunCMake/CommandLine/RunCMakeTest.cmake

Tests/RunCMake/CommandLine/copy_input/d1/d1.txt

Tests/RunCMake/CommandLine/copy_input/d2/d2.txt

Tests/RunCMake/CommandLine/copy_input/d3/d3.txt