Export targets and make sure they can be used by other projects

[jmcarcell]

BEGINRELEASENOTES

• Export targets and make sure they can be used by other CMake projects

ENDRELEASENOTES

This allows other programs to use the targets without having to rely on some variables that are set like LCIO_LIBRARIES and so on but it's backwards compatible (a clean up would be nice...). This will help with key4hep/k4geo#290 and iLCSoft/LCCD#7 that will have to be adapted based in this PR.

[jmcarcell]

I made a few changes not to modify INSTALL_SHARED_LIBRARY since I realised after that this is used in several repos and added a SIOTargets file in case it's builtin so that it installs it like it does when using standalone SIO. I think this should be ready now and not interfere with other packages.

[jmcarcell]

Any comments about this @andresailer @tmadlener ? It's less changes than #140 that has been open since 2021 and probably will require more work

[tmadlener]

For me this looks good. Just for my understanding: Effectively this PR makes the builtin SIO install an SIOTargets.cmake, which then makes it possible for LCIO to simply find it via find_package? Resp. it makes packages that depend on SIO (but maybe not LCIO) able to find SIO, even if it is builtin?

[andresailer]

When changing key4hep/k4geo#298 to use LCIO::LCIO and building on top of this PR everything looks nice.

[tmadlener]

Also works in my local build of the stack.