Allow project relative workspace paths

While I find this to be a very odd practice that I would not recommend, it seems that people like to open workspace directories as subdirectories of the BuildStream project itself.

The one benefit I can see to this is that if the workspace is a project relative subdirectory; then one could move the project to a new directory and carry the workspace along with it as a subdirectory.

But for this to work without breaking the workspace/project relationship, BuildStream would have to support serializing and interpreting the workspace directory as a project relative path.

To make this work:

• bst workspace open must check if the specified path is a subdirectory of the project
  • If this is the case, then we can avoid calling os.path.abspath() here
• When interpreting a workspace directory loaded from the project state directory (project/.bst/...), we should be initially using os.path.join(project_directory, workspace_directory); this can be done without consideration of whether the path was initially relative or absolute, due to the behavior of os.path.join()

Update by @toscalix : a proposal that covers this case has been sent to the mailing list.