Exporting and packaging prebuilt libraries in cmake - c++

Question

instead of asking a question directly, I'll expose my use case and the way I tried (but failed) to solve it.
Say I have:
3 shared libraries A, B and C
A require B and C
A comes with a set of headers
That's it, no extra information, it's provided by a vendor and not possibly subject to any change (modernization/cmake packages, etc).
A should always be packaged with B and C. I should only need to link with A and cmake should transitively link with B and C.
Now, I'd like to make it more "modern cmake" friendly and by able to:
First usecase: Create a repo containing these libs and calling add_subdirectory() from a parent project.
First usecase: Create a package (say debian pkg . deb) containing the relevant AConfig.cmake AConfigVersion.cmake and ATargets.cmake. Then a simple system install of the pkg and a find_package() should to the trick.
What has been done:
I tried using INTERFACE IMPORTED library and INTERFACE.
Because I want to support packaging the libs INTERFACE IMPORTED can't be used (you can't install it as far as I know/tested).
INTERFACE is working fine for the first usecase, using add_subdirectory(), headers are found, everything links, but because the user may not have at this point, the shared lib in is path, he can't run the tests for instance.
Then comes the export part needed to make the shared libs available and to make find_package() work. I succeed to export/package the libs A B C, the headers for A and the files needed for find_package().
But when in a client library D, find_package(A REQUIRED) finds the lib (no messages such as "Could not find a package configuration file provided by "A" ") it doess NOT create any target I can link on. I took a look at the generated ATargets.cmake:
# generated stuff before
add_library(A::A INTERFACE IMPORTED) # This is cannot be used at all, does not create a target I can link on unless I remove IMPORTED
set_target_properties(A::A PROPERTIES
INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include" # that's fine
INTERFACE_LINK_LIBRARIES "A.so;B.so.1.0;C.so.1.0" # that's not fine I need ${_IMPORT_PREFIX}/${CMAKE_INSTALL_LIBDIR}/ before each libs like it did for the headers
)
# generated stuff after
Note that if I remove the IMPORTED in the add_library of the ATargets.cmake and remove the namespace stuff, the target A is correctly accessible in any client cmake project using find_package but it'll NOT link correctly probably because A.so (and B and C) is not referenced using ${_IMPORT_PREFIX}/lib/A.so. I tried adding ${_IMPORT_PREFIX}/lib/ before all libs in INTERFACE_LINK_LIBRARIES, removed the IMPORTED keyword and namespace stuff and guess what, it works perfectly... Now, how do I do that without the trick.
The content of AConfig.cmake.in is:
#PACKAGE_INIT#
include(CMakeFindDependencyMacro)
# Add the targets file
include("${CMAKE_CURRENT_LIST_DIR}/ATargets.cmake")
# check_required_components(#PROJECT_NAME#) # It is commented because unless I apply the fix specified earlier (IMPORTED and namespace removed), during the find_package call I get:
#################
# CMake Error at /usr/lib/cmake/A/AConfig.cmake:8 # (check_required_components):
# Unknown CMake command "check_required_components".
#################
That's pretty much it, the rest of this post is implementation details. This code below can be used to solve use case 1 but the export package and cmake config/target files are not correct.
add_library(A
INTERFACE)
add_library(A::A ALIAS A)
target_include_directories(A
INTERFACE "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>"
"$<INSTALL_INTERFACE:include>")
target_link_libraries(A
INTERFACE "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/lib/A.so>"
"$<INSTALL_INTERFACE:A.so>"
"$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/lib/B.so>"
"$<INSTALL_INTERFACE:B.so>"
"$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/lib/C.so>"
"$<INSTALL_INTERFACE:C.so>")
#### install
install(TARGETS A
EXPORT ATargets
RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR} COMPONENT A_Runtime
LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR} COMPONENT A_Runtime NAMELINK_COMPONENT A_Development
ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR} COMPONENT A_Development)
install(DIRECTORY "lib/"
DESTINATION ${CMAKE_INSTALL_LIBDIR})
install(DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/include"
DESTINATION "include")
write_basic_package_version_file(AConfigVersion.cmake
VERSION "${PACKAGE_VERSION}"
COMPATIBILITY SameMajorVersion)
install(EXPORT ATargets
FILE ATargets.cmake
NAMESPACE A::
DESTINATION "lib/cmake/A")
configure_file(AConfig.cmake.in AConfig.cmake #ONLY)
install(FILES "${CMAKE_CURRENT_BINARY_DIR}/AConfig.cmake"
"${CMAKE_CURRENT_BINARY_DIR}/AConfigVersion.cmake"
DESTINATION "lib/cmake/A")
# Then some cpack stuff that is not affecting the work done earlier
In a client lib/cmake project after the installation of the generated package (generated by cpack):
find_package(A)
target_link_libraries(my_project PUBLIC/PRIVATE A::A) # Should bring in the headers and link with A B and C
Documentation:
cmake: create a new library target which consists of a prebuilt library
https://gitlab.kitware.com/cmake/community/-/wikis/doc/tutorials/Exporting-and-Importing-Targets
Possible to add an imported library to target_link_libraries that takes care of include directories too?
Exporting an imported library
https://discourse.cmake.org/t/how-to-control-import-prefix-in-exported-targets-cmake-list-file-generated-by-cmake/2291
Create relocatable package with proper autogenerated config cmake
https://discourse.cmake.org/t/exporting-packages-with-a-custom-find-module/3820/2
Thanks for reading/helping