`vt` installs `fmt` headers twice

[JacobDomagala]

Currently our install directory (with bundled fmt) looks like this:

We install fmt header to ${INSTALL_DIR}/include and ${INSTALL_DIR}/include/fmt-vt

Make sure we only install to ${INSTALL_DIR}/include/fmt-vt

[JacobDomagala]

Main culprit is the use of PUBLIC_HEADER

set_target_properties(fmt PROPERTIES
  VERSION ${FMT_VERSION} SOVERSION ${CPACK_PACKAGE_VERSION_MAJOR}
  PUBLIC_HEADER "${FMT_HEADERS}"
  ...
)

and the fact that both fmt itself and we install fmt target.

Following code is responsible for installing fmt headers under ${INSTALL_DIR}/include/fmt-vt

  install(TARGETS ${INSTALL_TARGETS}
          COMPONENT fmt-core
          EXPORT ${targets_export_name}
          LIBRARY DESTINATION ${FMT_LIB_DIR}
          ARCHIVE DESTINATION ${FMT_LIB_DIR}
          PUBLIC_HEADER DESTINATION "${FMT_INC_DIR}/fmt-vt"
          RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
          ${INSTALL_FILE_SET})

and this

install(TARGETS ${FMT_LIBRARY} EXPORT ${VIRTUAL_TRANSPORT_LIBRARY})

implicitly installs them under ${INSTALL_DIR}/include because target fmt::fmt has a PUBLIC_HEADER property enabled.