_project.py: Find project from workspace if outside of a project

Searching for the workspace file is preferred to searching for the
project file for the moment, because it avoids the circumstance of
the workspaces being inside a directory where a higher-level directory
may have project.conf (but may not be a buildstream project).

This will benefit from being revisited when support for multiple
projects owning a workspace is supported - if we're inside a project and
running commands in a workspace that refers to multiple projects, we
want the one we're inside, not whichever added it first.