Extend `WorkflowParser` to handle imports

[liamhuber]

@samwaseda moved in an excellent direction in #158. Since WorkflowParser is also an ast.NodeVisitor, we can pull a similar trick there. For conditional branches, we'll need to make a copy of the scope: ScopeProxy so that imports inside conditional branches aren't made available outside that branch, but I think it should all be pretty straightforward.

The goal is that we can parse nodes like

from flowrep.models.parsers import workflow_parser

@workflow_parser.workflow
def my_sneaky_wf(func):
    import inspect
    
    src = inspect.getsource(func)
    return src

Instead of requiring that the import be outside the workflow definition.