Update ProjectCodebase

[JonoYang]

* Remove root property and create root_resources
* Update walk to walk from root resources
* Update tests

[JonoYang]

@tdruez Is this something we want to keep?

https://github.com/nexB/scancode.io/blob/update-ProjectCodebase/scanpipe/templates/scanpipe/project_tree.html
https://github.com/nexB/scancode.io/blob/32110916537ea0facbc9bdeebc19b33b20b249fa/scanpipe/views.py#L736

[tdruez]

@tdruez Is this something we want to keep?

Nop, removed in 3cf6394

[tdruez]

@JonoYang This seems related to #378
What's the status on #378? Is it still relevant following this PR?

[JonoYang]

@tdruez

#378 addresses the same issue I had before, where I wanted a root CodebaseResource created whenever we create a Project such that I can do depth-first traversal of a Project codebase. I made changes to create a root CodebaseResource with the name and path of ., but there were issues with creating a VirtualCodebase object from scancode.io JSON results where the we could not visit any other Resource than the root Resource in the VirtualCodebase because no other Resource starts with . (VirtualCodebase/Resource walk logic uses the parent path to find its children). The changes I made in this PR work around this issue by just walking from each Resource in the root of the project, rather than from a single root resource. This way, we do not need to perform major changes to how Project/CodebaseResource and commoncode Codebase/VirtualCodebase/Resource works.

[tdruez]

@JonoYang could you add an entry with those implementation details in the CHANGELOG about the changes?