fix: define root/dd4hep hints as packages config

[wdconinc]

Briefly, what does this PR introduce?

This PR moves the explicit hinting for root and dd4hep dependents into a separate package file, to ensure this is handled consistently by being defined in a single place.

[Copilot]

Pull request overview

This PR introduces centralized configuration for root package variants by creating separate package configuration files (packages_root_with_opengl.yaml and packages_root_without_opengl.yaml) and updates two spack environments (xl and ci) to use these configuration files instead of explicit variant flags.

• Creates two new package configuration files for root variants (with and without OpenGL)
• Updates xl and ci environments to use centralized root configuration
• Removes explicit dd4hep variant flags from xl and ci environments

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
spack-environment/xl/spack.yaml	Includes packages_root_with_opengl.yaml and removes explicit root and dd4hep variant flags
spack-environment/ci/spack.yaml	Includes packages_root_without_opengl.yaml and removes explicit root and dd4hep variant flags
spack-environment/packages_root_with_opengl.yaml	New file defining root package requirements with OpenGL and WebGUI enabled
spack-environment/packages_root_without_opengl.yaml	New file defining root package requirements with OpenGL and WebGUI disabled

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