[manifest] licensed doesn't find license content from file comment headers

[jonabc]

The manifest dependency source is intended to be used in situations where a package manager is not available. Usages so far have been for c/c++ projects where dependencies are vendored into the project as source copied from another project.

There is significant friction in using this source when vendored source code doesn't contain an explicit LICENSE type file. This is not uncommon for c/c++ projects where license and copyright information is often copied directly into each file as a comment header. The current workflow for updating dependencies in those projects looks like

1. run licensed
   1. if licensed finds dependency updates, it updates the version and clears out the license text / metadata for each updated dependency
2. re-add license text / metadata as needed for each dependency and commit
3. re-run licensed to ensure everything is up to date 👍

Neither licensed nor licensee currently have the ability to understand license/copyright information as a comment header. licensee explicitly states that it's not a project goal to support header comments, which means that fixing this should be handled in licensed.