Cannot resolve dependency if module name differs from project name

If you have a dependency where the module name differs from the project name, you will get the following message on the Resolve Dependencies page:

I can't find "content" in the repository. Either the repository is out of date (you can update it here), or it's permanently broken for your version of drupal. Try checking its project page.

To replicate this issue:
1. Install a fresh copy of Drupal 6
2. Install Plugin Manager
3. Use Plugin Manager to install the Filefield module
4. Click to resolve dependencies when prompted.

Filefield depends on the Content module. PM searches for a project called Content, but the Content module is in the project CCK. Projects that package a lot of modules under one project name are all going to have this problem.