How to declare dependency on other ROS package for launch files

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I'm creating a package that exclusively uses launch files from other ROS packages. In other words, I'm not creating any nodes I'm just making a package to conveniently group and launch nodes from other packages. How am I supposed to declare those other ROS packages as dependencies, if I do at all? Is it in the package.xml using the <run_depend> or some other tag such as <exec_depend>?

I've checked REP 140, REP 127, the package.xml wiki entry, and Roslaunch tips for large projects and I'm not 100% how to do it. From what I can tell, REP 140 says to use <exec_depend> for

...packages providing shared libraries, executable commands, Python modules, launch scripts or any other files required for running your package.

which, to me, sounds like if I want to use the launch files from another package then I should declare that package as a dependency in package.xml using <exec_depend>.

So, again, the question is how am I supposed to declare those other ROS packages as dependencies, if I do at all?

Edit: I should have tested the <exec_depend> tag before mentioning it because I get the following error:

The manifest (with format version 1) must not contain the following tags: exec_depend