Unify and refactor the discover step

[therazix]

After the merge of #4391, there are still several areas in the discover step that could benefit from further unification and refactoring.

• Dist-git handling: Both discover plugins currently have their own implementations. The fmf discover plugin also introduces additional keys, such as dist_git_init and dist_git_extract. It would be beneficial to move dist-git handling into the discover step itself and unify the implementation and the keys where possible.
• Handling of local repository content: When using a local repository, the fmf discover plugin copies the fmf root into the test directory (/var/tmp/tmt/run-XYZ/plans/plan-name/discover/default-0/tests) before the initial test discovery. The shell discover plugin creates a symlink to the worktree, so no files are copied into the test directory.
• keep_git_metadata key: This key is currently supported only by the shell discover plugin. Would it make sense to extend this functionality to the fmf discover plugin as well? At the moment, the fmf discover plugin always preserves git metadata.

There may be other keys or behavioral differences worth unifying, or things I may have missed. Feel free to mention them here.

[LecrisUT]

To add to the list, the path handling of the test_dir is inconsistent between different discover with artifact-type: (url|git) etc. E.g. with git type the file tree is truncated from the path value upwards and test_dir points to that value, while with url it does not do the trimming. If we can define what the most common expected data should be then we can converge the behaviors.