path: move handling of "default" (*) suboptions value inside __init__
With the introduction of `path://` scheme the handling of default value will need to be subtler. We do simple code movement first to clarify the future changes.
Differential Revision: https://phab.mercurial-scm.org/D10260
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true