Encountering this issue, in the process of upgrading a D7 site to D8, it took me a while to realize that there might be a slightly different question that often is more relevant:
How would one install D7 with modules, similarly to how one would install D8 with composer?
drush make was the answer for me. Unless I'm mistaken, that tool was contemporary with D7 to a much higher degree than composer was.
An easy way to get started is to install drush 8 (make is removed in newer versions) using composer, and then adapt
drush/drush/examples/example.make.yml to your needs. For more details please see the documentation.
Clearly this is a bad idea for production systems, but that goes for creating new D7 instances today too.