Use FILE_SET to install the headers in /include together with the edm4hep library

[jmcarcell]

BEGINRELEASENOTES

• Use FILE_SET to install the headers in the top folder together with the library. This also adds them in the BUILD_INTERFACE, something that a simple INSTALL doesn't do.
• Bump the CMake version of the LCG stacks
• Simplify finding ROOT, don't do environment variable manipulation in CMake

ENDRELEASENOTES

FILE_SET requires CMake 2.23 so the version of the LCG stacks has to be bumped. I discovered that it also has the nice property that it complains if any of the sources that are attached to the library are not installed.

[tmadlener]

How important is this for the new Key4hep release?

It seems the only thing that is missing are a few format complaints from pre-commit / clang-format. Which either haven't been caught by previous versions of clang-format, or the file wasn't checked. Anyhow, this should be fairly straight forward to fix.

[jmcarcell]

How important is this for the new Key4hep release?

It seems the only thing that is missing are a few format complaints from pre-commit / clang-format. Which either haven't been caught by previous versions of clang-format, or the file wasn't checked. Anyhow, this should be fairly straight forward to fix.

For the release it doesn't matter. About the complaints, we can ignore them since I get different ones with the new .clang-format so I'll make them when I push the file (probably after the release not to create conflicts in the last PRs). So I think on my side this is finished.

[jmcarcell]

Right now the pre-commit CI works fine but there are changes, probably due to a change in the version of clang-format. Since the .clang-format is going to change anyway, I'm going to merge this as it is and then make the formatting changes in a different commit