Component core included globally and also in header mode

[wsxarcher]

When using fmt in header mode in my project and using cpack to create an installer, fmt core component gets detected as a component to make into the installer even if it's not needed.

add_subdirectory(fmt)
target_link_libraries(test
                       fmt::fmt-header-only

To remove the component from cpack I needed to do this in my cpack settings.

get_cmake_property(CPACK_COMPONENTS_ALL COMPONENTS)
list(REMOVE_ITEM CPACK_COMPONENTS_ALL "core") # remove fmtlib lib component (core)

Maybe would be better to call it fmtlib_core as components in cmake are global when included as subdirectory (?).
And if possible the component name should not be included when using in header mode.

(Related to #4276)

[vitaut]

The header-only mode is not recommended and we shouldn't change install behavior based on it but a PR to rename the component to fmt-core is welcome (fmtlib is the name of the github org, not the project). I think it can be done unconditionally since the name is global.