add_dependencies: Distinguish target v. file dependencies in error (#14050)

When called with a non-existent LHS target name the user may be trying
to add file-level dependencies.  Clarify the error message to explain
the difference between target-level and file-level dependencies.  Point
the reader at the commands and options needed for the latter.
Merge topic 'clarify-add_dependencies-error' into next

de13d68 add_dependencies: Distinguish target v. file dependencies in error (#14050)