"WARNING: couldn't find metadata for <pkg>" when using "license-report" command

[jtdaugherty]

I'm trying out cabal-plan's license-report command and I happened to notice this in the output:

WARNING: couldn't find metadata for mattermost-api-50200.14.0

The mattermost-api package is a direct dependency of the executable on which I am running license-report. Its source is located in a Git submodule of my source tree and the package is listed in cabal.project. Is there any way that I can ensure that cabal-plan will also include mattermost-api and its transitive dependencies in its license-report output?

Thanks!

(CC matterhorn-chat/matterhorn#740)