If the IMPORTED target name is provided in form of <pkg>::<pkg>, I think it is better to keep the case of letters <pkg> the SAME, which means libssh2::libssh2 or Libssh2::Libssh2.

For example:

• fmt::fmt
• spdlog::spdlog
• CLI11::CLI11
• Catch2::Catch2

Therefore,

• Scheme 1: libssh2::libssh2, modify:

  L400 of src/CMakeLists.txt

  install(EXPORT Libssh2Config
    NAMESPACE libssh2::
    DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/libssh2)

  L405 of src/CMakeLists.txt

  export(TARGETS libssh2 NAMESPACE libssh2:: FILE Libssh2Config.cmake)

• Scheme 2: Libssh2::Libssh2, modify:

  L218 of src/CMakeLists.txt

  add_library(Libssh2 ${SOURCES})

How about that?