add_library/add_executable: allow local alias to imported targets

Help/command/add_executable.rst

Help/command/add_library.rst

Help/manual/cmake-properties.7.rst

Help/prop_tgt/ALIAS_GLOBAL.rst

Help/prop_tgt/IMPORTED_GLOBAL.rst

Help/release/dev/alias-local-imported-target.rst

Source/cmAddExecutableCommand.cxx

Source/cmAddLibraryCommand.cxx

Source/cmGeneratorExpressionNode.cxx

Source/cmGetPropertyCommand.cxx

Source/cmGetTargetPropertyCommand.cxx

Source/cmLocalGenerator.cxx

Source/cmMakefile.cxx

Source/cmMakefile.h

Tests/RunCMake/GeneratorExpression/RunCMakeTest.cmake

...eratorExpression/TARGET_PROPERTY-ALIAS_GLOBAL-check.cmake

...ke/GeneratorExpression/TARGET_PROPERTY-ALIAS_GLOBAL.cmake

Tests/RunCMake/alias_targets/RunCMakeTest.cmake

...RunCMake/alias_targets/get_property-subdir/CMakeLists.txt

Tests/RunCMake/alias_targets/get_property.cmake