Add missing metadata like homepage, description in DirectoryListedSource

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