Add missing metadata like homepage, description in DirectoryListedSource #111

We should add a get_default_package class method with hardcoded metadata in each DirectoryListedSource, and get_package_info should build upon this default data with download_url, released_date, etc.

@classmethod
def get_default_package(cls):
"""Return a Package object populated with default for this data source."""
return Package(
  homepage="",
  description="",
  vcs_url="",
)