The mix source will detect mix.lock lockfiles, and parse their contents to enumerate dependencies.
The mix CLI tool itself isn't used to extract dependency data, and no project compilation occurs during license extraction, but your dependencies should be present in your project deps/ directory, as is conventional with Elixir projects.
Your project should contain a deps/ directory containing the dependency sources. You can download your dependencies (which are defined in mix.exs) using mix:
mix deps.get
This will create your mix.lock lockfile if needed.
Be sure to re-run this command whenever your mix.exs dependencies change to update your lockfile and dependency sources.
Because mix deps.get does not generate mix.lock entries for :path dependencies (nor are they stored in deps/), license information is not extracted from them.
If you need to extract license information from dependencies from another local directory (that is a Git repository), consider using a :git dependency with a file path in your mix.exs.