feat: add an option to not include copy_to_directory output in runfiles

[kormide]

Fixes https://github.com/aspect-build/bazel-lib/issues/748.

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config: no
• Suggested release notes appear below: yes

Added an attr add_directory_to_runfiles to copy_to_directory to not include the output directory in the target's runfiles.

Test plan

• Covered by existing test cases