[cmake] Only add the Core dependency to dictionaries when Core is a target

[jmcarcell]

This Pull request:

Changes or fixes:

#23207 made every dictionary with a MODULE depend on Core by inserting Core into ARG_DEPENDENCIES, which then ends up in the DEPENDS of the rootcling custom command. This works inside ROOT's build, where Core is a target, but downstream projects only have the imported ROOT::Core, so the bare Core is treated as a file and the build fails. For example, when building Garfield++:

make[5]: *** No rule to make target 'Core', needed by 'GarfieldDict.cxx'.
make[5]: Target 'CMakeFiles/GarfieldDict.dir/depend' not remade because of errors.
make[4]: *** [CMakeFiles/Makefile2:232: CMakeFiles/GarfieldDict.dir/all] Error 2

This adds the Core dependency only when Core is a target, so the behaviour inside ROOT is unchanged and downstream projects work as before.

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes a regression introduced in #23207

[github-actions]

Test Results

    24 files      24 suites   4d 1h 29m 37s ⏱️
 3 883 tests  3 880 ✅ 0 💤 3 ❌
83 112 runs  83 108 ✅ 0 💤 4 ❌

For more details on these failures, see this check.

Results for commit 449f28b.

♻️ This comment has been updated with latest results.

[pcanal]

Looks good to me.

[jmcarcell]

It would be great if this is merged today together with #23626 so that we have dev3 builds tomorrow, a few of the issues we had should have been solved.

[pcanal]

@linev Could you take a look at the failures of test-stressgui-xvfb on opensuse16: https://github.com/root-project/root/pull/23618/checks?check_run_id=112499176495 and https://github.com/root-project/root/actions/runs/37449769482/job/112280590348?pr=23618

[andresailer]

@linev Could you take a look at the failures of test-stressgui-xvfb on opensuse16: https://github.com/root-project/root/pull/23618/checks?check_run_id=112499176495 and https://github.com/root-project/root/actions/runs/37449769482/job/112280590348?pr=23618

Seems unrelated to this PR: #23632

[andresailer]

Could we get a review / merge today so we can see further tomorrow?